I just took the Porthcawl Insurance. They were helpful and friendly on the phone, and sound as though they know what they're on about! (Which always helps)!
The quote was pretty reasonable and gives an agreed value for everything except the engine, which will be market value in the event of a claim. (I believe most, if not all policies have this).
It covers everything you'd expect including trailering, launching, recovering, storage, underwater fittings, electronics, legal expenses and accidental damage, etc, etc.
Let's just hope I never need to use it.