Hi DavieG,
Several things . . . Quads are the most dangerous thing with an engine so insurers hate them. Usual rules apply, dealerships usually have the best introductions for insurance deals as they need to shift stock. As quads are in the 'ATV' category the NFU Insurance Company at Stratford on Avon will insure them, you will probably have to make a good case for being associated with the land if not an actual farmer though . . . they do not see themselves as insurers of choice for chavs revving the latest fashion accessory past the urban chippie!
Whatever you do though make sure you are signed up on a full riding course. Not the crap EASI intro but something that actually takes you through the vices of these things like the BORDA Stage 1 or the LANTRA course - at least a full day. You may find that like 'pass plus' with a car license having a proper training certificate will get you lower insurance.
As for rates, I don't know, ours is included on a trade policy so you'd have to ask. Please be careful though I mean it about these things being dangerous. Google 'Yamaha Quads Accidents' if you don't believe me!
KRs Funforus