have been doin some ridiculous mileage recently though, i did a couple thousand miles last week and i dont tend to go slow around the windy roads of mid wales anyway work or not, dont bother worrying about it anymore as alot is for work and gets subsidized anyway. 
Slightly off topic but do you claim the Mileage Allowance Relief back?
I keep looking at it as I do a lot of miles a year for work but never want to get the tax man involved in my finances, 
But if your company is paying you 12ppm you might be able to get tax relief up to 40ppm which may be better in your pocket than theirs.
More details here http://www.hmrc.gov.uk/incometax/relief-mileage.htm
It all depends on what car option you are on for your company. This is how ours works.
Option A - You have a company car which is leased by the company and you keep for 3 years (budget on monthly payments is determined by CO2 emissions). You get 12ppm for a petrol engine.
Option B - You are given £3500 a year in the form of car allowance which is taxed and added to your salary. You get 40ppm for a petrol engine.
Option C (what im on) - You get £5500 a year in the form of car allowance which is taxed and added to your salary. You get 12ppm for a petrol engine.
I don't do MASSIVE miles so went for the higher initial allowance. Which basically gave me £350 a month to pay off any sort of finance.
HTH.
