Update: Renault CS called me and have upped their offer. It's been upped to 75% which I have accepted. The dealership have also knocked a good chunk off my 12mth service price. I know some got higher (rja_cup getting 95% from RUK and 2.5% from his dealer) but I've accepted based on the age/mileage when compared to other cases, and having weighed up the risks of going down the Sale of Goods Act route.
I still think that this is something that Renault should look to sort FOC for anyone suffering what is a common problem, and I could easily see someone else willing to take this a long way. However, whilst this isn't the sort of hassle anyone wants, it's an especially bad time for me, so I'm willing to accept the goodwill gesture I've been offered and hope that this is an end to the problems.
Thanks for all the messages of support as well guys