I think that you can claim for previous year's mileage without having to file a tax form. A few years ago I wrote a nice letter to the Inland Revenue quoting NI number, etc and gave a breakdown of mileage travelled, allowance paid and then the total of the difference between the permitted allowance at 40p per mile against what the RAF paid me. In the good old days before JPA you could go into Gen Office and get a print out of your milagae claims; I am not sure if that is the case now
I received a cheque very quickly which changed my views on the tax man!
Well worth putting pen to paper in good old ISS style and getting some cash back.