I needed insurance just the other week actually, on my derv, to drive it home. Had to get 48 hours policy, and (gulp) I was already part way though the drive when I realised my existing insurance didn't cover me. I stopped in the motorway services on the M6, umped on the internet, and got cover within 15 minutes, with the cover note on my iphone. Result. Paid £34 for the two days if I recall.
I think it's always gonna be very expensive prorata for short term policies.
I went through tempcover.com - don't know if you've tried them?