i think you'll find these sorts of things are difficult to enforce, as so many people these days have caravans which they keep at home. council's are reluctant to get involved as they would end up having to pursue too many people, costing them a lot of money.
if you have a friendly word with your neighbour and explain about selling your house, etc., and bring up again about them saying it's not staying on the drive(there may be genuine reasons why it's still there) i'm sure you can sort it out. people are usually very reasonable if you keep things friendly and don't start threatening legal action. ask them in for a cuppa and bring it up then.