Dutch Bike

remove some of the insulation and twist old and new wires together lots - over an inch or two, but not so it gets too fat. use electrical tape to cover the join and hold the insulation as well, then carefully feed the new wire in while pulling the old wire out. Eventually, unless you're too forceful and break the join, the new wire will be happily sitting in place.

If there are removable rubber grommets, remove and slide them up the wire so there's more wiggle room and less friction. replace once the new wire is in place.
I will try that as well. Bolting the (cheap, chinese) bottle dynamo more securely fixed the problem for all of 10 seconds. Riding off, smug in my well lit, practical glow, all was well and then it just faded out. Back to square one.

After earthing issues, I just don't know. Cos of the well-documented kickstand / rear light issues, the red plastic cover is getting knocked off every other time, so it could a broken bulb / loose connection at the back end, which has never worked anyway.

I think I'll treat myself to a new dynamo and lamp for christmas.