Hi again
Okay, I think I've tracked down what causes it in my system. It seems that it only happens when I am at a gate that I have not edited using the COUATL menu and then if I use the editor to change any parameters of the parking spot, e.g. radius, baggage loaders, etc. the pushback doesn't happen. However, if I park in that spot again, i.e. shutdown P3D and then park there again and all subsequent occasions it works fine. This happens with all planes, default and 3rd party.
I hope that proves useful information. In any case, now I've worked it out it isn't really a problem anymore. The reason it took so long to find is because I tend to fly from different airports with different planes a lot.
Thanks
Andrew