I haven't come across it but I agree that the manager should have a discussion with the employee and negotiate when they will give them a response. The employee may need to arrange childcare well in advance so she might have a deadline in mind. Otherwise, the employer has 3 months and I'm not sure what grounds you would have to refuse.
In all likelihood flexible working laws will change sometime this year so the issue around resubmitting within 12 months may not be valid.

It is not unreasonable for an employee to want to plan and many employers would be grateful to any employee giving such advance notice. The statutory scheme encourages employees to apply well in advance by including a 3 month decision window.
If none of the statutory reasons for refusal apply then why not grant it on a trial basis?.

In reply to David:

Thank you Elizabeth and David for your replies.

In reply to Elizabeth:

I have given up second guessing when/if the Govt will change any employment law to employee's favour
I tend to agree that 15 months is very much long term planning for most SMEs. The one ground that might work is potential structural change (if it is true)
I am very sympathetic to childcare issues but 15m is probably too long unless you are an employer with a totally sympathetic approach and sufficient size to accommodate.

The flip side to this is surely with 15 months' notice, you can plan to make this change work? Finding someone with the right skills to work 2 days a week might not be very straightforward, but it doesn't feel like it's impossible with that lead time? From the employee's perspective, it doesn't feel unreasonable to want to give more notice rather than less.

This is a curious one!

Obviously, it's always better to have a dialogue *before* the submission is made to cover off these conversations but there's no way to actually stop an employee from just submitting their formal request. And nothing to require that their request start within the immediate future.

If I had to find a legal justification for refusing, it would probably have to be "planning changes to the workforce", on the basis that in any period of more than 12 months there's almost inevitably going to be changes to the workforce. But it's a pretty weaksauce argument and it certainly wouldn't stand up in an ET if push came to shove.

I think Nina's point was my own first thought: i.e. with this much notice, there should be more than enough time to accommodate the request.

By all means see if the employee will have a dialogue and discuss a re-submission closer to the date but, in practice, this will disadvantage her and really provide more opportunities for the employer to be able to establish a reason it cannot be agreed, so she would in some respects be a fool to do so.

This is actually really quite clever on the employee's part. I wonder if this is the start of a trend?

This happened to me in an SME before where as Peter said, it would be considered long-term planning.
We had a chat with them, thanked them for making us aware of their plans so early but encouraged them to hold off until nearer their return. Not because of business needs but because their circumstances might change. Such as returning on a different date or their childcare situation.
In the end, I think they did change their hours but the non-working day was different to the original plan and the manager was able to anticipate so it all worked well.