Well, I took it apart tonight and as I sat there stumped, I suddenly realised that one of the tiny washers at one end of the worm was in the wrong place! It was on the motor side of the black lug that the worm sits in, so I believe there was too much play in the worm. I have now put it in the right place and stuck it back in the car.
All sounds good, apart from a loose bit on my exhaust somewhere (there's always something) and still kicking in when it should and lighting up the Vis Indicator as it should.
Result!!
