We've gone with Towergate Mardon (via Michael Stimpson) as well. I have a house insurance policy which includes the personal possessions away from home and that covers laptops, jewels (like we have any!), gizmos etc. wherever they are. But it doesn't cover the nb's removable items .... personal possessions left on the nb when we're not there. So we have contents cover added to the nb Towergate Mardon insurance policy.
In reading all the paperwork carefully in advance, the 'key facts' document from TM states: "You are not insured for the person in charge of the craft will be over the age of 18 years of age ....". It's a misprint, of course, but it reads as though anyone over the age of 18 is not insured! Michael Stimpson is getting the leaflet corrected and says noone else has noticed it despite years of sending out the leaflet. So I guess all the other TM insured boaters haven't read their documentation properly, hmmmmm???