We use Worldwide Insurance (
http://www.worldwideinsure.com/) having checked that motorcycling is covered and having received the following response:
"Jean@Customer Services" wrote:
"I am pleased to confirm that our policies do all provide medical and emgerency cover for motorcycling holidays. This means that you would be covered for the cost of treatment or repatriation following injury whilst motorcycling. However, there is an exclusion under Personal Accident which means that you or your estate would not receive the lump sum compensation payout in the event of your permanent disablement or death resulting from a motorcycling accident. In addition, the Personal Liability cover within travel insurance does not apply whilst riding the motorcycle. I hope this is of help to you. "
It's the medical/emergency cover which is important to us, the personal liability would be covered by the bike insurance and the death or disablement cover isn't very high anyway.
The exchange of emails was some time ago though, probably best to check they've not changed their policy terms.