If the expenditure is revenue, the timing of the tax
deduction will follow the charge against profits in the commercial
accounts drawn up in accordance with GAAP. Therefore if that charge
was spread over the primary period (which may be the case,
especially if the costs are recovered by being specifically
factored into the rate of interest implicit in the lease) you
should follow that for tax and not allow a full deduction in the
period when the costs are incurred.
If practice among leasing companies diverges on this point
you might wish to seek accountancy advice.
