https://www.bmw-service.co.uk/
Check out this website, just enter your reg and mileage. If your lucky then you'll have the remainder of a service pack, I would have thought most M5's were bought with one. @ £999 for 60k miles or 5 years you'd be silly not to.
You'll also find the the book price for the servicing, so you know how much you're expected to pay outside of a service pack.
Personally I'd favour the BMW extended warranty as you know what your dealing with. Yes it's going to be a little more expensive, but at least you know your covered and unlikely to have to battle with them to payout.