Nigel
As everyone seems to be shying away from ths one - here goes - there are so many variables with this it is impossible to give a definitive answer - I think that is why no one wants to put their head above the parapet.It seems that you have a "car" provided by the company. Dependent upon the mileage it might be better to take a taxable increase in pay, buy your own vehicle and claim 10p a mile fuel off your employer and claim the balance from HMRC. Obviously your employer would have to go along with this and in doing so he would have to be convinced that it would, at least, be cost neutral -- employers NIC becomes an issue here.
A car is described at
http://www.hmrc.gov.uk/manuals/eimanual/EIM23020.htm
Vans are treated differently -
http://www.hmrc.gov.uk/manuals/eimanual/EIM22725.htm
A pick up is likely to be a car for tax purposes .....
If you want to contact me personally
bob.jones25@btinternet.com
Regards
Bob