I use TARDIS from Autosmart. Very effective.
petrol
Some interesting posts here. As I started the thread, the suggestions made have been useful. I have given WD40 a go and this certainly does work, spraying on to a cloth and then rubbing on the tar. I like the idea of methylated spirit and can see that that should work; will have to give it a try. Gradually I shall work through several of the ideas to see which is easiest to apply.