Thanks for all the helpful replies, and I now know who wrote that article in RIB int Dave! Special thanks to Brian C; I have been in contact with Britannia and they cover trailers up to 8m in length (excluding the tow vehicle) provided it is fitted with a 50mm tow hitch. The trailer in its own right is covered for breakdown - I was told that they will recover your trailer whilst you can drive on to your destination
A discussion like this made sure that I knew the right questions to ask (I hope!!)
Thanks again chaps, looks like a result for RIBnet