Mark - it sounds like a manual box, not an auto. I had a similar problem on mine a few years ago and it was the nylon bush on top of the gearbox, where the levers from the gear-shift actually operate the linkage into the gearbox. If they are not broken or dislodged, then spray lots of spray grease all over the top of them and wiggle the gear-stick to get the grease into the gaps. If they are broken or really worn out, then it's a bummer as the parts cost around £4, but it takes three hours to pull off all of the bits and replace them. Best if you can do it yourself - look down the gap between the battery and the coolant bottle with a torch and see if you can see what moves when someone else tries to select the gears and you should see the linkage parts down there.
Helpful Mark - not related
October 2008