18. For the purposes of this Part, the following rules apply:(a) related persons are deemed not to deal with each other at arm’s length;
(b) a taxpayer and a personal trust, other than a trust described in any of subparagraphs a to d of the third paragraph of section 647, are deemed not to deal with each other at arm’s length if the taxpayer, or any person not dealing at arm’s length with the taxpayer, would be beneficially interested in the trust if section 7.11.1 were read without reference to subparagraphs b to d of the second paragraph; and
(c) where paragraph b does not apply, it is a question of fact whether persons not related to each other are at a particular time dealing with each other at arm’s length.