that's an odd system that you have Dave? Most employees get to claim x pence per mile if they are using their own car for business use (which is higher than just fuel cost as it is supposed to cover wear and tear as well).
I'm guessing the main issue is that for ease of calculation your company is paying out x as an approximation for the price of petrol and that the recent rises means this is not covered. You should probably raise it to the accounting department - I doubt it would be career-threatening to do this.
As there is an element of personal use you will also be taxed as a benefit in kind (ie a taxable perk of the job). The only way around this is to use it only for business use (even a commute will count as "personal" and it must be kept at business premises outside of working hours).
I'm guessing there is no choice of company car use or cash?
Bit more detail here.
http://www.bizhelp24.com/accounting/...any_cars.shtml good luck! Chi