Here is a link to an old article (c2007) I found while reseaching the problem.
Electric Bike Hub Motor - How to Replace a Hall-effect Sensor
Electric Bike Hub Motor - How to Replace a Hall-effect Sensor
Thanks for the link aseb, I missed that thread and the video when looking on ES.There's a nifty little tester I got from Lyen that tests the hall sensors, and lots more.
Ebike motor/controller hall/phase/throttle tester $29
If you check the posts at ES you'll find posts about changing them, what different Hall Effect sensors to get etc. btw Lyen sells them too, and is very helpful and knowledgeable.