Just to report back in case anyone else treads this way....
All back together now and it's nice and quiet now. The only really awkward jobs were getting the two-piece plastic cover back on after I refitted the motor unit and getting the old bearing off the motor shaft. I didn't have a puller that was small enough to get behind the pound-coin sized bearing. All of my tools are designed for cars. Even Mrs T's Uncle who restores vintage motorcycles didn't have one. In the end I used two small levers and various bits of wood to protect the plastic case. If anyone else tries this method remember to keep the shaft held into the motor so it doesn't try and pull out with the bearing. We achieved this by wedging one end of a long baton under a lowered motorcycle lift (complete with 1956 Moto Guzzi and Mrs T's Uncle on board) and the other just touching the top of the shaft to hold it in. It took a good pressure with the levers to pop the bearing off.
The plastic covers are just really fiddly, there are three loose rubber seals that all have to be in place at the same time as the two parts of the covers. Definitely not a job for anyone who gets impatient and throws things!
We eventually decided on thin epoxy sticky pads to hold the Hall sensor circuit board (the type used for mirror tiles) after some heat testing with an old credit card and a tin lid! This was all Mrs T's work since she has better eyes and thinner fingers.
The only odd thing is there is still a kickback on the pedal if you press it until the motor engages and then take your foot off it. It's not really the way you would ever ride the bike so I'm just assuming it always did it for now.
All in all it's quite a do-able job if you are fairly mechanically minded. Certainly cheaper than a new motor at £7 odd plus postage for the bearing.
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