Not entirely sure but I'd request reducing your hours with job sharing as the solution for the school. You might already be aware of the procedure for requesting a change in contract but I was advised that the school need to see that the situation would work for them. If you have someone who you could job share with (already employed by them) then I'd request reduced hours and explain how the job share would be a successful solution to staffing your 'missing' hours. I think there is more of a chance of your request being accepted if the school can see that they wouldn't be at a disadvantge. Good luck!
It's worth requesting it (Theogriff on TES website forum has a good standard letter to use) and see what happens.
With lots of schools shedding PPA staff in favour of HLTAs etc and lots of newly qualified teachers desperate for some experience, you might be surprised at how easily the role filled.
3 different people were interested in my share when I wanted to work 3 and a half days. The head thought it would be difficult to find someone for 1 and a half days, but there were people there who wanted it.