My missus works for a broker who deal with all sorts of insurances with various underwriters, if you're totally unsure go see a high-street broker to get an idea of what coverage you need and rough prices, then either buy through them (they'll have certain brokers they put certain things through) or hunt down a better deal online. You could also contact the underwriter yourself to cut a deal but quite often the high-street broker will have a better rate.
The most important thing is making sure you have the right coverage, most should offer comprehensive cover which will not include things like racing, off-peste etc!