To be honest you want a reputable company that is likely to pay out rather than the cheapest.
Things to do to keep the cost down:
1) Stay within the conventional home trade area (British Isles & Brest to the Elbe)
2) Keep your boat in a marina or ashore over the winter
3) No racing
4) No singlehanded sailing
5) push up the excess to as much as you're comfortable with
After that, shop around.
In the medium term try to get an RYA certificate or two. Some insurance companies even take account of the Dazed Kipper cert.
Edited: and just to make sure there's no misunderstanding from the order of the posts, I think GJW are reputable.
I used www.craftinsure.co.uk when I started (still do - very competitive). Never had to claim yet, so I can't comment on how readily they pay out, but I take the view that the main reason I want / need insurance is for the third party component, then theft of the boat (rather than gear), then to cover damage I may do myself so the payout ease is of lesser importance to me.
I keep the boat in a marina so theft risk is generally reduced anyway.
Good luck, but mostly enjoy your first few seasons (I know I have) /forums/images/graemlins/smile.gif
I've been with Craftinsure for three years now. As I've only had to give them money there have been no issues. I'll wait to see what happens when I ask them for some back!