I do not have to pay the company back for private mileage.
I will be doing about 18000 miles a year which will be traveling to work and other mileage, non will be actual business mileage.
It doesn’t matter how many miles you do, the taxable amount is an ‘allowance’, which means even if you do 1 private mile paid for by the company, you pay the government a set amount.
Taken from http://www.macintyrehudson.co.uk/content/taxcentre_taxcard/vehicle.html#fuel:
If the employer provides any fuel used for private journeys and is not re-imbursed for the cost, the 2009/10 tax bill for the fuel is: £P11d value of the vehicle x 23% x (income tax band)%
So if you pay 40% tax, and your car is worth £18,000, you will pay £18,000 x 23% x 40% = £1,656
This is set to double in 2011 with the lates budget announcement by Alastair Darling, so I would think very carefully before allowing an employer to pay for any private mileage!

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