Under which code do you show petrol card given to an employee for unlimited use?

There is no log book kept, employee just uses this card for all petrol, tolls etc. I have recommended that they start using a log book, but it will only be from recently. So for the 2013 tax year, they would not have this logbook yet.

My feeling is that you put it under travel allowance (even though the amount is different every month). Employee gets taxed on @ 80% of the allowance on a monthly basis and if no logbook at assessment, the remaining 20% will be taxed.

Any thoughts?