Playing devil's advocate ... the issue was simply that the car had a slightly erratic idle? Bearing in mind the age (17 years) I suspect that wouldn't - legally - make it 'unfit for purpose'. What mileage has the car done, and how much did you pay? What sort of warranty did it come with?
Regarding the loom, I assume TM spotted that this was on the way out and really ought to be replaced. Unless it was actually causing a problem now I doubt you could expect the seller to pick up the tab.
Having said that you have nothing to lose by - nicely - approaching them to see whether they will play ball. Whatever happens I hope you get the car sorted out quickly. TM Motors are very good, my SL has been there a couple of times in the past.