10(1)For the purposes of the principal section, a company is an associated company of a participator (being itself a company) making an election under that section if—
(a)throughout that part of the relevant period in which both were in existence one was a 51 per cent subsidiary of the other and the other was not a 51 per cent subsidiary of any company; or
(b)each of them was, throughout that part of the relevant period in which it was in existence, a 51 per cent subsidiary of a third company which was not itself a 51 per cent subsidiary of any company.
(2)In this paragraph "company" means any body corporate and section 532 of the Taxes Act (subsidiaries) applies for the purposes of this paragraph.
(3)For the purposes of this paragraph the relevant period ends on the date on which the election in question is made and begins—
(a)in the case of an election relating to expenditure incurred in the first claim period of the field of origin, on the date on which any part of that field was first determined under Schedule 1 to the principal Act; and
(b)in the case of an election relating to expenditure incurred in any other claim period of the field of origin, at the beginning of that claim period.