OK. Here we go - after a hiatus. 
Pat was very good about the lost kit and sent me another one straight away. Would you believe it? A couple of weeks later, the first one turned up. I've sent the spare one back, before anyone asks. 
Anyway, I've checked the Weaver tower and the sprockets have no cracks. What happens, is that the chain hangs up on the teeth end then jumps as it releases - in both the Weaver and the P&D!
Here they are, side by side . . .
I had made up a brass idler, which stops the chain wrapping itself around the top sprocket. That stops the lashing action which was causing jumpy movements in the loco at Notch one. The P&D one has the wrapping around syndrome as well, so I could change the idler over - or keep the Weaver one.
That's what I've decided to do. I'll have spare sprockets is I ever need them. The seller gave me a bunch of spare chain, so I'll be OK with all the spares I've got. 
Pat sent me an upgraded motor as well. The motor in the Weaver was struggling with the Loksound decoder (more like the other way around), so I'll see if the new motor improves things.
I've laid 5 metres of track on my new layout, so I'll have somewhere to test it out.
Thanks for your interest.
Wish me luck.
