Hi Steve,thanks for the advice.I checked the movement of the selector tower and it felt "gritty". Hopefully with new oil it should be ok.Do you know the location of the filler and drain points?.My engine is an ATL and I believe the box is a GRJThis sounds like water has got into the gearbox, caused by the collecting cup (which catches water that comes through the vents in the bonnet) being knocked out of place.
If that is the case, you need to reposition the cup above the gearbox, and you'll want to get the gearbox oil changed.
Martin, the short answer is I don't. This thread (despite being about a petrol model) might point you in the right direction: https://www.a2oc.net/community/index.php?threads/gear-box-filler-plug.30086/.Do you know the location of the filler and drain points?.My engine is an ATL and I believe the box is a GRJ