I ordered a pickup mainly in "poke and hope" mode in the absence of any other ideas, I must admit it was more expensive than I expected, the £125 quoted turning into £161 when you add VAT, post and VAT on the post! The next problem is that it doesn't really fit on this bike. Here is a photo of the OEM pickup in place:
As you can see, there is an additional bearing for the camshaft, which bridges the pickup, and on the end of the camshaft there is a special nut with a square hole in the end, which engages with the tacho cable drive. The next picture shows the NLM pickup with the bearing next to it:
The bearing won't fit over the pickup, which is much deeper than the original:
The advice from NLM is to discard the bearing assembly. I'm kind of reluctant to do that because I can't help feeling that the bearing was put there for a reason. Has anyone else been down this path? Or can anyone throw any light on why that bearing was fitted on the later model bikes?
In the meantime, something weird! When I removed the original pickup, I took it over and plonked in on my bench, then went over to the bike to pick up my tray of screws, etc from the pickup. I now found the pickup was sitting on the bench in a pool of water - where did that come from?? To cut a long story short, the plastic sleeving around the two red wires was full of water, some of which had now run out - could that be causing my problem? So, trying not to disturb anything too much, I unsoldered the red wires from the pickup and measured the resistance between them - it read around 1M ohm, though it would double or halve if I even touched the sleeving. I now pulled the wires out, fully expecting them to be damaged or to have perished and cracked insulation, but the wire looks perfect, smooth and flexible. I've now re-assembled it with new wire and sleeving but I haven't had a chance to try it yet. I know the wiring was a bit leaky (electrically leaky I mean, though it seems to be leaky in the plumbing sense as well!), though 1M ohm wouldn't affect its performance - maybe it got worse when warm? Who knows, as soon as I get a chance, I'll try it.