I would take matters up with
through the dealership and insist that they bring it up to the standard that it purported to enjoy when you bought.
I don't think they are within their rights to make you keep the car if it didn't fulfil its function at any time. There are certain assumptions made when buying a car that has only done 20k miles and it is reasonable to expect it to have good brakes that aren't damaged in any way otherwise there will be safety issues. But, as they replaced discs and pads FOC there is no longer an issue.
The Megane has a known issue with
and your dealership can tell you which batch of
were substandard. They have a "Yellow OTS" on them which is a scheme created by Renault to iron out these issues at their expense. If the car has suffered from this problem previously, it should have had all the
changed when it first happened. Obviously this was not done on your car or they would not be causing problems still. If they were done, they may still be within warranty anyway as all work /repairs done are guaranteed for 12 months by the dealership. So you need to negotiate with your vendor and reach a mutually acceptable solution.
I can understand someone saying "caveat emptor" if you bought this privately but there is more protection when buying from the trade. At the end of the day, lawyers never agree anyway, that's why they end up argueing it out in court. Of course I bow to Mrs Velsatisfied's superior knowledge on the question of "are you within your rights to ask for a full refund?" but I wouldn't half give my vendor a hard time, if that's what I was after.
Negotiation is easier/better than confrontation. So, good luck.