There's been one or two threads about this in the past. Most of those that do have insurance have found that where possible an extension to their household policy is the cheapest way to arrange it.
Most insurance companies will insist on a heavy duty lock, although this should be a priority anyway.
I got £1200 of cover on mine, both for when in the shed and also when out being used, subject to being locked up with a cable lock when not in use. It cost £23 for the year simply added on to my home and contents insurance with Norwich Union
Some household insurers will not accept electrically assisted pedal cycles (probably do not know what they are), but eSure, run by the Bank of Scotland will.