They just pay my fuel for work journies, 50p a mile which will go down to 14p a mile if I go on the lease scheme
yes but you claim the tax relief on the difference between HMRC's approved rate and what your work pays you....
HM Revenue & Customs: Travel - mileage and fuel allowances
so for the first 10,000 miles that you drive: 45p-14p =31p x 10,000 = £3,100 (you get tax relief on this amount, so if you're a higher rate, its 40% of £3,100, which is £1,240) this is claimed through self assessment and will improve your personal allowance code.
Then if you do >10,000 miles per year...its 25p-14p =11p x ??? ...etc etc
This facility has nothing to do with work btw. I've been using this allowance for many years.
It can also be backdated too! ....you need a mileage log -simple excel spreadsheet will suffice.
also, I think for this allowance to apply, it has to be for travel which is not your 'normal' place of work (eg your fixed office). i work from home, so every journey is claimed.
hope that helps