Thanks.After my local dealer had given me their best price, I used carwow to see what I could get elsewhere. I showed them the best offer I received (which was around £3,000 under what they had offered, plus it included mats) and I asked them to match it because I was buying the car either through them or elsewhere. They had to make some phone calls to their head office and Mazda, but they matched it (of course!). So although the sales wasn’t registered through carwow, it was worth using it to get a better deal.
Sounds as though there really isn't anything to lose going via this route. I shall certainly give this a try.