I've used the ETA for a while now and they have been great. It's bike specific insurance so might be a bit more expensive than an add on to contents insurance, but I've had no messing about when claiming and they don't seem to have any stupid requirements in the policy.
They also provide a load of extras now, such as "breakdown cover" and a taxi home. Not sure exactly what breakdown cover for a bike entails but might turn out useful one day. http://www.eta.co.uk/insurance/cycle/summary
Bikes I've insured with them have been from £500 to £1000, but after my last one was stolen, I haven't insured my new one (£2500) as I can't really afford it. I just went for a **** off abus dlock and a little motion sensor alarm, always store the bike inside the house and never leave it out of site.