I found an EU enquiry web site Signpost services
which mostly answered my question, but their reply also contained some general information that others might find useful:
Dear Mr Thompson
Thank you for your inquiry.
Unlike for vehicle driving licences, there are no EU rules that provide for mutual recognition of navigation certificates. We cannot therefore provide you detailed advice on the formalities involved. However, we can provide you with the following general advice.
Since your Advanced Powerboat certificate is unlikely to be fully recognised in France, you will need to obtain an International Certificate of Competence before you leave. You can find out more details on how to obtain one by calling the RYA on 0845 345 0370 or browsing the RYA’s website: http://www.rya.org.uk/Cruising/faqs....tegory=1020414
You will also need to ensure you have the following documents with you when you sail to France:
• Registration Document (SSR / Part 1);
• Ship’s Radio Licence;
• Proof of insurance (& translation if required);
• Proof that VAT has been paid;
• Proof that your vessel complies with the EU Recreational Craft Directive (RCD);
• VHF Operators Licence.
You will also need to comply with certain formalities if you want to operate your vessel for commercial gain.
Firstly, you need to register your vessel with the port authorities of the port where your vessel will moor in Brittany.
Secondly you need to register with the commercial vessels with the French office of Maritime Affairs (Affaires maritimes).
Thirdly, you will need to obtain a commercial permit from the French customs office of the port where your vessel is registered.
Finally, if you wish to fly the French flag, you will need to re-register your vessel in France.
The French customs can be contacted on 00 33 (0)820 02 44 44 or by email on email@example.com
and have a website where you can find out more information (only available in French): French Customs
The French office of maritime affairs can be contacted by email on firstname.lastname@example.org
also have a website (only available in French): The French office of maritime affairs
We hope this answers your inquiry.
Citizen Signpost Service