Sale of Goods Act say anything should be of merchantable quality, but proving the defect existed 2 months ago when you bought a 130k miler is going to be nigh on impossible.

I would expect that you should have been given written details of any warranty on your car and without that you are reliant on goodwill, so best be nice to the dealer!

EDIT:
Q2 on this seems to suggest that for the first 6 months it's up to the dealer to prove the car WASN'T faulty at the time of sale:
http://webarchive.nationalarchives.g.../page9010.html

More up to date (but the same) stuff here:
http://www.lawgistics.co.uk/used-car...cond-hand-cars