By chance I happen to have my houshold insurance in front of me. Under the Occupiers Personal and Employers Liability section (£2,000,000 limit) there is an exclusion which amoungst other things says, we will not cover you for boats and craft designed to be used on or in the water, other than:
- those propelled by oars or paddles
- pedestrian controlled toys or models
There is therefore affiirmative cover for model boats to the tune of £2m third party liability!
If anyone looks at their houshold policies they will see similar language. There is also similar bits throughout the policy but qualified with "other than" again confiming the third party liability section will respond to a model boat accident.
There does not appear to be any other qualifying language so in the absence of any, or any specific exclusion there is full cover for any model boat for free.

Enjoy, Geoff