After reading later comments about Comfort now being OK out the MPV issue, I decided to call Comfort again this afternoon but spoke to a different member of staff the second time.
This time this chap said they knew of the MPV issues but they are happy to quote and cover without need to get the V5 changed?
So it looks like its down to the luck of the draw on who answers the phone as to what the policy is. If the person you speak to starts to talk about altering V5 details for your Cali, then call back later and hopefully you will get a positive outcome.
Unfortunately we then hit another stumbling block due to wanting to insure the Beach for business use and carriage of own goods, which they were not sure about, so I am waiting a call on Monday for them to see if it's possible.