Sky were always cheap for me, but shite when I wanted agreed value. For some reason they were fine for Yoof - living round the corner (at the time), working at same place, and younger with less NCB - but I think I got a retard on the 'phone and that was it!
I'm with Greenlight now as they were hungry for the business, and a sensible (IMO!) agreed value was no sweat.
Brentacre, HIC, Adrian Flux etc. are all worth trying, as are any other specialist brokers you can think of. If you want the best price be prepared for a lot of ballache ringing round everyone to find out who likes your risk and wants your custom. I've rarely stayed with the same insurer on the Polo for more than 3 years to keep my premium as low as possible.
Others experience may differ - so ring around!