I've not had to do it but the general rule of thumb seems to have been that Renault give the gearbox a life of 100,000Miles. If you've done, for example, 21,000Miles then you would be expected to pay 21% of the repair and they would foot the bill for the remainder. It hasn't been an exact science though unfortunately and some people have had bigger contributions than others but a few people have come back on after speaking to RUK quoting figures based around this 100,000Miles idea. It's a bit of a strange one as on the Clio the gearbox is a non-service item so technically it should last the life of the car and not just ''x'' amount of miles. I'm not sure if it's the same story on the Twingo but I'd guess that that box is a non-service item as well...