A2QFI
It depends on the amount that you are claiming back. HMRC have specific allowances for mileage that are currently 40p per mile for the first 10,000 miles per annum and then 25p per mile thereafter.
If you have claimed 40p per mile then there will be no liability, if you have claimed less then you will be able to get a rebate. If more, then there may be a liability.
Please note that I am not an accountant or in the forces but I do get a nice lump sum back annually because my company pays less than the 40p p m.
I believe that there was a similar post on this last year so a search may bring up further info.