Dave
I would suggest you do a search on Warranties to see people's thoughts. However, here is a potted history.
When the 6 was brought into the UK, CCC were offering a 3 year warranty and through an agreement, also provided the warranty on Ralliart cars. CCC were toatlly in control of these warranties, setting the terms and conditions. More importantly, CCC altered the terms AFTER the cars were sold. There are 2 primary areas that have changed. They no longer allow any upgrades or tuning and brakes are no longer covered!! Due to this, Ralliart now give their own warranty on new cars supplied by themselves which is fairer.
Unfortunately, this does not help those who have been caught out. This has happened to both customers of CCC and of Ralliart. Because Ralliart's hands were tied by CCC, some people have become p****d off with Ralliart not meeting the warranty claims and not allowing upgrades. To make matters worse, some people were allowed stage 1 upgrades early on.
So the issue on a secondhand 6 is whether thewarranty is worth paying the usual premium that is comanded by official cars. Simple answer seems to be that if you want to tune the car, it is not worth it. If you want to keep the car as standard, then it probably is worth while. However, it might well be worth checking out Ralliart because I believe that they give their own warranty on secondhand cars they sell, as might other dealers.