In reply to James Malloch:
Last time I bought a new car and had one to sell, when I was changing insurance onto new car I asked insurer about short term cover for old one for the same purpose. They simply ran cover on both vehicles at a cost that was about double my normal rate on the old car per fortnight... They were happy to do it in one or two week increments. I suspect they might have asked questions/pulled out if things dragged on but as I sold within a few days I never had to extend what I originally agreed with them.
Seemed normal to them. I thought it would be an issue but it was far easier and cheaper than either short term cover or new policy without any NCD that you then need to cancel.
Time has passed so you may have missed opportunity but speak to them - its clearly a common scenario. My wife and I have 2 cars and a policy each, the insurer obviously felt that the risk wasn't high as we couldn't drive all 3 at once and mileage would remain similar....