Just to chuck my tuppence in here...
I was previously with Aviva, last year was ~£900, this year the standard renewal letter was £730... a little haggling got that down to £603. Comparison websites were getting me figures of around £500, with Admiral being the cheapest.
The cheapest that is, until I saw QuoteMeHappy.com - I got it for £301, with £40 cashback from TopCashback (also available on Quidco), so a total of £261 !!!!
Here's the silly part. QMH.com is a wholly-owned subsidiary of... wait for it, Aviva ! The policy is of course under-written by them - same registered office, same FSA#, I believe when you call to do a claim you're actually talking to Aviva staff - it's just a different brand, yet I got my renewal for 50% of their best renewal quote - even less once i get the cashback (which was tracked within an hour of taking the policy) !
Regarding the policy, compare them side by side and the wording is almost exactly the same. Well worth a look, IMO.
They'll only insure you under certain conditions, and i believe you MUST have some sort of NCB.
The one potential drawback for some people is that they are online ONLY. Any enquiries are done online, documents uploaded/downloaded etc... but claims can be done over the phone.