Hi all,
Just thought to update this with my investigations.
Yes, a good many travel insurance policies either exclude motorbikes, only allow 125s or have other concerning wording/premiums/disallowances.
On the bright side, travel insurance held with your bank seems to be good quality - e.g. Lloyds bank travel insurance is underwritten by AXA which seems not bike-unfriendly. http://www.axa.co.uk/insurance/travel
http://www.axa.co.uk/assets/documents/axa.co.uk/personal/insurance/travel/travel-policy-booklet.pdf - Policy wording
I then noticed that AXA won silver in the Travel Insurance Awards 2008. The winner in recent years seems to have been, to my surprise, the Post Office.
Reading their insurance policy, they explicitly mention riding motorbikes.
http://www.postoffice.co.uk/portal/po/jump1?catId=62900708&mediaId=63300708
http://www.postoffice.co.uk/portal/po/content1?catId=89500762&mediaId=19500189 - Policy wording
Their policy seems to be underwritten by Fortis.
Hope this helps someone 
Alex