The fantastic support of this Forum strikes again!!! particular thanks to Chas for his help. I learned a valuable lesson, so I'll play my part and pass it on.
When re-installing the motor, to get the right level of tension on the belt:
1. Make sure the gearbox end of the motor is sitting ON the supporting lip of the mounting bracket (variously referred to as the "fence", "card" or "blade")
2. Loosen the mounting bracket so you can angle the motor in pointing slightly down; this gives you enough play remaining in the belt to fit it onto the outer gear cog.
3.Twist the motor round inside the mounting bracket, when you re-fit it, so the red wire output from the motor is horizontal and pointing immediately backward - this is the tightest part of the adjustment.
Fitted the gearbox and motor this way, and I'm back in business!