Martin (1992: 385):
Collocation has been included as a text forming resource in the English Text model in order to allow for the possibility that there are inherent expectancy relations among lexical items at the level of lexicogrammar which make a contribution to texture independent of the semantic relations predicted by field.
 To be clear, collocation is a text forming resource in SFL theory, not just in the English Text model. It is a form of lexical cohesion, a resource of the textual metafunction. See, for example, Halliday & Matthiessen (2004: 576-8).
However, Martin locates collocation within the experiential lexicogrammar, despite the fact that it is concerned with (second-order) relations within the text rather than with the (first-order) construal of experience, and without demonstrating how it relates to other experiential systems, such as transitivity at clause rank.
 Lexicogrammar and semantics are two perspectives — wording vs meaning — on the one phenomenon: content. They are two levels of symbolic abstraction in a relation of identity. By definition, they are not independent. All wordings realise meanings.