s 205 Reduction of annual investment allowance and first-year allowances
(1) If it appears that a person carrying on a qualifying activity has incurred expenditure on the provision of plant or machinery— (a) partly for the purposes of the qualifying activity, and (b) partly for other purposes, any annual investment allowance or first-year allowance to which he is entitled in respect of the expenditure must be reduced to an amount which is just and reasonable having regard to the relevant circumstances. (2) The relevant circumstances include, in particular, the extent to which it appears that the plant or machinery is likely to be used for purposes other than those of the qualifying activity in question. (3) In calculating for the purposes of section 58 the balance left after deducting a first-year allowance, a reduction under subsection (1) is to be disregarded.