£299 by itself doesn't sound a bad figure. However, if it covers a car for 3 years then since the 500 is on 2 year/18,000 mile intervals that'll basically be one service, unless you do under 9000? a year in which case it gets an annual service, which the service plan might or might not cover.
My biggest problem is that it's with Hartwell. Having family who've bought several Vauxhalls & Fords from some of their other dealerships, I'm of the opinion that you'd be better off taking the 500 to one of your other local dealers (like Windsors of Heswall or Forward Autos, St Helens) if you want to maximise your chances of being completely satisfied.
Hartwell are good for getting a decent deal when buying a car if you haggle as they're a big group & seem to be able to knock money off easier than the smaller dealers, but for service I'd be surprised if you end up completely satisfied with them.