Dynamo cutting out after a couple of seconds This sounds like I've an electrical issue, but no idea of next steps.

I have a Gazelle Tour Populair with what I believe is a Busch &amp; Müller LYT Lumotec Retro front dynamo-powered light.

When starting to move the light comes on, but quickly cuts off for the rest of the journey.

The shop have already replaced the light with a new one, and same issue.

Could it be a power build-up somehow?
You could try measuring the voltage the dynamo outputs when spinning the wheel. Strip two thin wires on both ends , insert them into the dynamo plug (plug which is usually present on the hub to which the wire going to the light Is attached), attach a multimeter and spin the wheel. If the voltage drops to zero/drops considerably after a few seconds whilst still spinning the wheel the problem is not with the dynamo and it's probably a short circuit/ground or bad electrical connection somewhere.

Example of what a dynamo plug/ connector might look like:
https://i.stack.imgur.com/fQyr8.png