p87 or self assessment

[jaskhan]

Good Morning Guys

I know there is a lot of topics on here about P87 but cant find anything that gives me my answer,
I have a new client who needs to claim back mileage for the 3 past tax years.
I know all the workings out etc , and a separate P87 needs to be filled out for the separate tax years however 1 of the years goes above the £2500.00 of allowable expenses you can claim on the P87 and a self assessment should be filled out.
I read somewhere that you could just put it all in a letter to hmrc.
Has anyone else had a situation like this or know of the best and fastest way to get payment from hmrc ?

Many thanks