TQVM for that, I bought a special tool because I have 3 BBS01Bs so I will get some use out of it.
I was unsure how to remove the seal without damaging it.
(37 sec in) was clearest, used a small flatblade screwdriver without damaging seal.
Found the PAS circuit board and PAS disc. No scoring or visible dirt, but cleaned with isopropyl alcohol on qtip.
The PAS circuit board had some smears of white silicone straying 3mm onto board, so removed this.
Re-assembled, and sadly problem was still there, but now I know how to get at PAS.
Plan: Replace PAS board (have spare) (can I remove silicone and access connector via final drive - or do I have to remove controller?).
If that fails to work, replace controller (on order), and if no joy, replace whole motor.
Not fixed yet, but at least I didn't make it worse, and picked up useful skills on the way.