[broken link removed]
Page 14, Section 5.8.2:
Equally an adjustment will be required where a car is
for some other reason not available for private use for
part of the tax year, for example, where an employee is
working abroad for an extended period. In this case, a
car provided to an employee will not be regarded as
available for private use for that part of the year in
which the employee is outside the State for the
purpose of performing the duties of the office or
employment, provided the following conditions are
met:
n The employee travels abroad without the car
n The car is not available for use by the employee’s
family or household during the employee’s period
of absence outside the State.
AFAIK if you hire a car it is a business expense and does not add to you business miles.
Basic you need to work out the number of days you had (available) use of the car as a percentage of the year and then reduce each "Mileage Thresholds" band by that percentage, from there you can work out how much BIK you owe. I your trips were same day or over night stays it then there is no way they would count as an 'extended period'.
Towger