Many thanks for your reply, Gareth.
You make some very valid points which I need to consider carefully. However, it annoys me that it's illegal for a manufacturer to say a warranty is invalid just because a car hasn't been serviced at a franchised dealer, yet they won't provide the information needed to service it elsewhere! I suspect they would lose in court if this stance was challenged (not that I plan to do so!) and it surely can't be legal to invalidate a warranty if the servicing details are not on the VAG system.
As you suggest, I'll check out prices at other dealers but I'd still like to have the option of going elsewhere, so if anyone else can help with the service schedule for my car, I'd be grateful.