First, check your existing motor insurance policy. Many provide 3rd party cover for driving any motor vehicle (which is not owned or hired by you) which your licence covers you to drive. You might already have the necessary cover. (If you've any doubts about whether the cover includes motor cycles, ring the insurer).
If you're not already covered, ring your insurer and ask whether they'll provide a temporary extension to your cover.
If not, take out a one day policy:
http://www.dayinsure....torbikeInsurance.aspx
Chris