I also insured with GJW when I purchased my Nc39 back in late 2009. However I found that they repeatedly increased the premium each year by over the cost of inflation and I had not made any claims. When challenged they reduced the price to my last annual premium.
I am now with Y Yacht insurance who I have found to be more competitive. Again never made any claims but they have had a good comments on various posts on the YBW forums. Shop around.