Thanks all.
I had another go today with a few tools. Turns out it was running with too little weight (1.65g, alpheta 2) and too much antiskate. I ended up covering the top of the tonearm lifter with a thin layer of blutak. Works great now.
Audio technica AT6006R tonearm lifter
I've been using one of these for a few years on my garrard 301/sorane sa1.2 and it's been great.
Recently I tried to set one up on a friend's rega RP10. It was quite tricky to place because of the hole in the plinth in the ideal position, but managed to get it working OK eventually.
The big problem is when the lifter activates, the antiskate force pulls the arm off the lifter and it falls off back onto the record, with the associated awful noise and wincing.
Any bright ideas on how to fix? I've checked the anti skate and it's set to about 1.5 on the scale. It really needs something to increase friction on the underside of the arm. Maybe some rubberized tape or Blu tack?
Any ideas most appreciated.