Hugh, having just insured my 9m cabin RIB, Magellan Alpha, I would suggest you contact either Colin Curtis and Partners on 01752 664649 or Mardon on 01743 232688.
Mardon's price was the lowest and they were placing the insurance through Royal & Sun Allianc/DAS so the quality of the cover should be fine.
I chose to go with Curtis even though they were a bit more pricey because I felt they offered a more personal touch. They were placing the cover with Haven Knox-Johnson and Lloyds which is blue-chip stuff.
The only advice I would give is to make sure that whichever broker you use, do make sure that the cover is properly underwritten by sound insurance companies. I have seen some ugly results of brokers who have taken the money and not placed the cover. Get proof that you are covered!