This is the one for checking tax but it doesn't give you reimbursement rates.

http://www.vehiclelicence.gov.uk/EvlPortalApp/application;JSESSIONID_EvlPortalApp=CdqsSpqBAPG3Qj oobYHN4Ldij5iXfUchi9St8S4ErolyM4hqKFr4!-2138703071!-1906659212?origin=vnav_bar.jsp&event=bea.portal.fr amework.internal.refresh&pageid=Vehicle+Enquiry

Go to the home page and you should get something there.
In your own car you are entitled to 40p/mile for the first 10,000 miles in a year, then 25p for all other miles. If your employer pays less, you can claim tax relief on the shortfall. If he pays more, it is a taxable benefit.

If the car is a company car, some of the advisory rates were just increased:
Engine size Petrol Diesel LPG
up to 1400cc 10p 9p 7p
1401-2000cc 12p 9p 8p
over 2000cc 16p 13p 10p

Different tax-offices take different views on these but if your employer can prove you achieve poorer than standard fuel consumption (eg you need a 4x4 to get off-road to service mobile phone masts) and pays you above these rates there shouldn't be any tax to pay.