skaterboi,
For back claiming mileage expenses, you don't have to write the letter to HMRC that you so kindly copied, or send off proof. As an alternative you can simply complete and return a Form P87.
http://www.hmrc.gov.uk/forms/p87.pdf
It's a 4 page form but if you're just claiming for mileage then you only fill in about 6 or 7 boxes on the whole thing, many of which are repeated figures.
Disadvantages vs your method - you need a separate form for each tax year in question (although you could put them all in the same envelope). You need to be claiming less than £2,500 in expenses.
Advantages - no need to send off proof (nevertheless, get some proof and keep it for 7 years in case the tax man comes to check).
As for the cut off dates for back claiming, yes it is 4 years, but from the end of the tax year in question, so for tax year 2009/2010 the cut off date is 5 April 2014, for more info see:
HM Revenue & Customs: How to get allowances and reliefs - employees or directors
Simples....