From memory, without being ars*d to look up the guide on the Balpa website, what you claim is an Inland Revenue "FRE". This is a deductible allowance pa of £800 to cover all sorts of expenses you are recognised to incur. There is an additional £100 you can claim bringing it up to £900 total; I think this is for mileage. You can back-date your claims for the last 5 years if you've forgotten to claim previously (I think). PCs are not recognised expense. You don't need receipts to claim this FRE, but you will if you attempt to claim more than the £900 and can justify it within the scope of the table-of-expenses they allow.
Cue someone else to scoff at how vague and inaccurate I am being....but I'm away to take the tatties off the boil.