Just another update.
I have now disconnected and by-passed the BMS from within the battery pack, taken it out for a test ride, and sure enough - IT WORKS !!!
There is the odd intermitant hesitancy (cut out) particularly under full throttle, but I would now probably assume this is due to a bad cell?
At least it has given some idea to where the fault lies.
My only problem now is to find out what the AH is on the existing battery so that I can order the correct replacement.