No they don't. You might find the ACAS draft guidance (www.acas.org.uk/.../2024) useful. According to that this is what an employee must include in their request:

the date of the request
the change the employee is requesting to the terms and conditions of their employment in relation to their hours, times or place of work
the date the employee would like the change to come into effect
if and when the employee has made a previous request for flexible working to the employer

In reply to Elizabeth:

Thank you Elizabeth

Hi Claire,
I watched a webinar from MAKE UK yesterday and they stated that an employee is no longer longer required to explain the effect of the proposed change and how that effect may be dealt with.

Hi Claire 

Last month's CIPD podcast discussed the ins and outs of the upcoming changes. Well worth a listen... with links to other resources, too.

https://www.cipd.org/uk/knowledge/webinars/flexible-working-day-one-right/

In reply to Steve Bridger:

Just watched thank you so much Steve...all makes sense now!