Raising a grievance would be good because it then makes it WP as well as ' protected'
There are 2 ways forward:
Your way- inviting them and exploring
The usual way ie drag them into a meeting and outline the options and package in writing, giving them 10 days to consider.

I know what i would do but I don't work in Education.

A protected conversation as it is often called is designed to protect both parties including from a CD claim.

I apologise that there will be a certain amount of closing stable doors in this reply, but I think your account requires a postmortem:

He has come to me to say that he feels undermined, gaslighted etc etc.

Employees will often start a complaint with this kind of language: "I feel..." is a common way to begin. But it's unhelpful to any future resolution until we understand why they feel that way. You took this to the Head - who often isn't trained or qualified to manage these resolutions - and weren't able to get a resolution because, I suspect, the employee spoke in terms of their feelings rather than in terms of specific incidents where the Head had caused those feelings in them. Extracting this king of information is vital at an early stage because it helps to crystallise the problem. It might take only a single misjudged word in one conversation for all subsequent conversations to be tarnished by the feelings of hurt caused by that word. Identifying the word and the incident is then essential to resolving what can feel like (but isn't) a pattern of behaviour.

So when they say "I feel undermined" or "I feel gaslit" (it's a very trendy word and most people use it incorrectly), you need to push back a bit and find out why they feel that way. What happened? What was actually said?

This isn't just about investigation. It also helps the employee to reflect upon and articulate the reasons for their feelings which, alone, can help to move the process towards resolution.

I have had many conversations about this with the member of staff and have advised him of his options (including taking out a grievance if he could not resolve informally)

The moment they raised the issue with you, they had a grievance. The option of whether to resolve it formally or informally is the employer's, not the employees (although the employee is always welcome to express a preference). That you had many conversations without resolving anything is part of why you've ended up in the place you have. The time to start resolving the grievance is after the first conversation, not the point at which it ends up in a hearing.

He brought his wife.

I'm going to assume that she was also an employee, otherwise... no. Just... no. Why would you even let him do this?

we then eventually (with reluctance on the employee's part) arranged a meeting to discuss these concerns with the Head with me present

You aren't a professional mediator. Don't try to run a mediation session if you don't know how to do it. It isn't just about getting two people into a room to thrash out the issues. This was, I'm sorry to say, a car crash waiting to happen.

1. You never do mediation if both parties aren't on board.
2. You never start mediation by bringing parties together.

Always get a professional.

protected conversation

Although a written grievance would be useful for records keeping, there is clearly already a pre-existing dispute, so you can go directly to a Without Prejudice conversation, with its slightly stronger protections than a protected conversation.

The employee doesn't have to ask for one. You can just offer it. As with mediation, it requires informed consent on the part of both participants, so you can't just spring one on them.

However, I don't think this situation is unsolvable. I think you have an opportunity to wind it back, to talk with the employee properly to try to understand the actual actions and behaviours that led to their feelings of underminding and gaslighting. Then you can bring that back to the Head (without the employee being there!) and talk it through with them privately to see if they can, having been given the details of the actual incidents that affected the employee, perhaps see how they could have handled the situation differently or, failing that, dismiss the employee's concerns entirely if the Head either believes they didn't happen or didn't happen as described.

That way, you either have a pathway to an actual, realistic resolution in which they Head recognizes how they can deal differently with this employee in the future and amend their conduct (expecting an apology is probably a step too far) or you can dismiss the grievance as not supported by the evidence.

*At that point* the employee may ask that the grievance be formally investigated instead. Whether you agree to do so will depend on whether you think an investigation is likely to find anything to contradict your own conclusions.

With a little luck, you may restore the relationship to at least some extent and retain an otherwise perfectly adequate employee rather than spending a lot of money to get rid of one.

In reply to Peter Stanway:

This outlines things
www.masonbullock.co.uk/.../

If the school / governing body is prepared to splash the necessary cash then all well and good. Presumably if they're a competent teacher the employee concerned shouldn't find it too difficult at all to obtain more suitable work elsewhere so maybe no need for vast expense.

However competent they are as a teacher though, ultimately irreconcilable personality clashes in present workplace might conceivably be fair reason for SOSR dismissal but of course nowhere near that yet / that's the nuclear option......

In reply to David:

(crossed with Robey so none of that yet taken in )

In reply to Robey:

Wow so basically I've handled this incredibly badly. I have not gone into all of the detail in my post. I did have examples from employee, I raised them with the Head. The Head gave their view. The employee brought up these examples at this meeting. We did not describe the meeting as a mediation session. I disagree that an employee complaining about something should be treated as a grievance until they raise a formal grievance. I find your response extremely unhelpful but thanks for posting.

In reply to Jane :

Hi Jane

Clearly, a grievance has been raised informally and you've had a meeting to discuss it - albeit an informal one / not a formal grievance hearing, to which you exercised your discretion and allowed his wife to be there as companion. IMHO / in principle nothing wrong with any of that, and indeed it tallies with eg

www.acas.org.uk/dealing-with-a-problem-raised-by-an-employee

On here you'll get a range of opinions / perspectives - most will tend to be based on incomplete knowledge on the part of the responder of the full particular facts, so maybe best to make up your own mind objectively and dispassionately based on these - they'll also tend to be from a pretty expert and experienced standpoint.

In reply to David:

Thanks David

In reply to Jane :

Complaints that do not amount to a formal grievance can be interpreted as grievances by a tribunal depending on the facts established. If I complained of bullying sexual harassment and direct discrimination, it would be a foolish employer who did not deal with it as a grievance.

In reply to Peter Stanway:

Agreed.

I would start from the viewpoint that I wouldn’t be overly concerned about a constructive dismissal case. This is for two reasons, one they are really quite difficult for employees to bring, and two the more you go down a formal process, probably it’s even harder for them to be successful in a claim. Courts are wary in finding constructive dismissal when somebody is going through a formal process, unless the process is truly arbitrary.

You say there is no capability issue, but also that there are ongoing issues with their relationships with other staff. Well for me that a capability issue. And I would treat it as such.

As Peter suggests getting this into a grievance process, might be a way forward to provoking the conversation you want.

Do they have much service?

Fundamentally you’re going to have to engineer a situation where you can have the protected conversation ( providing the governors supported and are willing to pay for it) and I would be less concerned about how you do it, then it happening. The danger here is this just runs and runs and runs.

Just wading in with another aspect, does your School have a policy about dealing with complaints/grievances about the Head? In my School, for example, the policy is that written grievances (or where they become formal as they have not been resolved at an informal stage) are immediately referred to the Chair of Governors. I would suggest reviewing and following your policy carefully, and ensuring that actions that are well intentioned don't inadvertently cause you contractual issues down the line.

In reply to Nina Waters:

Indeed - most workplace grievance procedures will provide that grievances against particular individuals aren't in any way heard or investigated etc by them and even if they don't specifically so provide it's fundamental natural justice to avoid that kind of situation unless there's absolutely no reasonable alternative.

In reply to Nina Waters:

Yes, any written grievances concerning the Head go straight to the Board of Governors (Chair hears appeal)