Official version available from OU: You can find a TMA.sty published by the Open University at Suggested style for TMAs produced with LaTeX and in the OUSA LaTeX forum.
Note that the former says: You don't have to use this to produce a TMA in LaTeX, but it may help you to get started with a nice output.
The 2016 github version here is the same one, but since it was uploaded there, the OU has added it to its official site.