Paul, I use Glover and Howe of Colchester, Essex. http://www.gloverhowe.co.uk they insure both professional and amatuer photographers and amateur astronomers.
When I was shopping around I found that when the value of equipment was known and that it would be stored either inside the house or outside in a purpose built building the insurers would not include it on the normal household contents. I have heard that some people do, but I couldn’t get my C14 edge, and Paramount MEII and CCD plus. Hence I used Glover and Howe.
Hope this helps