A few thoughts on the legal side;
1, The garage's offer of a free MOT (or anything else) is NOT an admission, it is an attempt to placate a disatisfied customer and has no legal relevance.
2, The ONLY relevant document, is AMD's report, which the OP, for unstated reasons, does not appear to want to disclose.
3, The role of Trading Standards is to enforce consumer protection legislation, not to assist individuals (although that may happen along the way), they are also not relevant.
4, If, as a result of the dotorg love, the repair is undertaken for free, there is no claim against the garage as no loss has been suffered.
5, If the garage have not been given the opportunity to rectify, any claim is significantly weakened and giving them that opportunity is the only reason to write to them at this stage.
I'm happy to help with the recovery side, but you need to focus. Step one is the report.
AmD's report is to big to scan, not on A4 paper
however its contents ARE irrelevant to the case as i realized afterwards as GSS had the car the whole day before had and i had told them what the problem was, they would have removed the plugs to check i correct and there is a leak, and when putting them back, making sure they were to the correct settings..
there own report however is NOT irrelivant and has everything i need.
even with all the amazing support and result of the .org love its still costing me over 400 pounds to repare. (saving well in excess of over a thousand pounds)
the garage was given lots of chances to repair the damage and said to me several times that its not their problem and they wont be touching it again.