Try British Mountainering Council. in Manchester. you dont have to be a climber but they are very good value and are underwritten by Fortis.
If you go to the top of the page "Find a Question" and the enter "Insurance" in the "Keyword" box you will find loads of suggestions from passengers. Many of them would be suitable but get at least three quotes before making up your mind. ......Neil.
If you are not covered yet, try Columbus Insurance, found on Google.
Thanks for taking the trouble but we managed to find insurance after a good deal of enquiring. Thanks again.