As Chris says, it's a slog. I rejected a MKIV Golf a few years ago that was on contract hire with VW Finance. The dealer appeared to be on my side though which certainly helped. It was a three year agreement and they eventually gave me a brand new one about 18 months in. That was after I'd said no to an 18 month old replacement. VWs argument being that I'd had 18 months use out of the duff one. I countered by saying that may be the case but faults developed within weeks of me taking delivery.
At no point did I have to resort to legal advice or threats. I just dealt with VW via the dealer. The whole thing was initiated when I wrote a strongly worded letter to the head honcho of the dealer group.
Good luck Mark. I know how annoying it is when these issues aren't sorted.