There is a balpa leaflet from around 2007 which covers this in some detail. The definition of a temporary workplace was given as below. If you can get an up to date copy it would help as it is the basis of discussions between balpa and the taxman, and what the tax man will regard as reasonble expenses for the piloting profession.
"A temporary workplace exists if an employee
goes there to perform a task of limited duration.
However, if he or she spends over 40% of his
working time there, a temporary workplace will
only exist if it is anticipated that the employee
will be working there for less than 24 months."
It goes onto say:
"
Having established a temporary workplace, the
following travel expenses are eligible for relief:
1
All travel expenses between the temporary
workplace and the permanent
workplace, or any other business address.
2
Travel expenses between home and the
temporary workplace. Relief is restricted to
travel between the permanent and
temporary workplaces if the employee makes
a substantial stop at the permanent
workplace during his journey, although he
can merely pass the permanent workplace
without loss of relief."