Not trying to tout for your business, bobbysupascaff, but you and your mate and his accountant are treading a very thin line.
HMRC have a year from submission to come back on you. Just because it's alright now, doesn't mean they won't pick your file up randomly, and look further into it.
They have really tightened up on PAYE tax claims and want evidence. As Swifty says, without receipts etc how can you know you are getting as much as you should be... or worse too much and you'll get bitten.
Companies that pay out immediately as stonedrose1 says usually charge a premium for the privilege, as much as 35% and there is no way that this amount can be accurate, so is probably just overpaid tax based on your P60.
Bobby, you did use us in the past but have moved on, and that's fine. Just be careful is all. If we are the company you refer to, maybe you were only eligible for £200... our returns are accurate and correct, not guestimates that are over inflated to get you or us in trouble with HMRC.
'Nuff said.