I insured mine as 'camping equipment' on a policy from Hill House Hammond, together with the tent and all the gear too. The policy is meant for caravans and TTs, but they were happy to give me good cover on most eventualities for my tent, trailer and all the gear.
I found that my household contents insurance was very limited on the cover it could provide when my camping stuff wasn't in the house.
Have you tried the Camping and Caravanning Club - they seem to insure or know about just about everything there is to know - they've been really helpful to us over the years.
have you tried caravan guard ,i went to there site from here and got our trailer tent insured,very helpful young lady ,sorted it out for me ,give them a try.tel 01422 398698,or www.caravanguard.co.uk
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.