We use
http://www.worldwideinsure.com having checked with them and got the following reply:
"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. Jean Bonner-Williams "
Mind you, I shall check again before renewing it this year, just to make sure nothing's changed.
Oh and they are (or at least were) extremely good value too. Haven't ever tried to claim though.