Reduction is given
Yes, a reduction is due where the car is not available (ITEPA 2003, s. 152): "The cash equivalent of the benefit of the fuel is to be proportionately reduced if for any part of the tax year in question the car for which the fuel is provided is unavailable."
In accordance with s. 143, the car is unavailable if the day "falls before the first day on which the car is available to the employee".
In your case, the employment began on 6 December 2011 so the car was not made available before that date. So the benefit is reduced pro rata.
It is (almost) never worth incurring the fuel charge, however, so I suggest you get her to stop it as soon as possible. You may find that the extra tax she is paying is higher than the actual cost of the fuel (even without considering the position of the employer).
You legend! Thanks Ray :)
Why remove the question?
If you don't need/want further answers, just say so, or amend the title. But this thread may have been of use to someone else looking for similar advice. Without the question, the thread is meaningless (and unlikely to come up in a forum search), so may as well be removed.
I agree BKD
It's extremely annoying, and there is sometimes a different opinion anyway, so removing the question would remove the opportunity of getting more than one possible solution or answer.
I disagree, the car and availability is not relevant. TTP at position 69 clearly states the new tax will be applied from 1 January 2013 according to size. As a result all pleasure is now subject to taxation and the charge is the relevant tax rate multiplied by your size and the number of occasions you relieve yourself. I understand the penalties include small cages and corks.