I thought I would return to this now that I am sorted and hope that it will help someone in the future avoid "binning" a Pontec vacuum.
After much chasing around trying to find a replacement motor and that drawing a blank, I started looking at the alternative motors around.
The difficulty in that is that the physical dimensions are seldom ever quoted, and asking a seller to confirm sizes is a pain for them and not worth their time.
Anyway - in the end I took the plunge and bought an Aquavac 28001 motor - thoughts being that it would probably fit with adjustments... or silicon, depending on how you look at it!
I am pleased to say that it fits - tighter than the original motor, but without pressure on the case moldings. I had to remove a rubber sealing ring from between the base of the motor where it meets the plastic case (the intake) and reverse a rubber ring that is at the top of the fan case (a part that was on the original motor).
The appliance works great - only thing I may do is apply a light bead below the motor where I removed the rubber ring - but to be honest, it lifts water perfectly as it is.
Thanks for all the help guys - I would have had it in the skip without the comments on here.