Using a multimeter and some paperclips I tested each stage of the flashgun and the synchronised trigger mechanism in the camera and found that each stage of the system worked in isolation, but still no joy taking photographs with a flash. I think some of the connections were a bit loose due to corrosion.
I took the (very simple) flashgun apart and washed all of its copper connectors in spirit vinegar to make them red and shiny again. Most importantly, I found it was necessary to scratch the corrosion off the contacts of each bulb before use, using the handle of a teaspoon or similar.
Now I can enjoy reasonably reliable retro flash photography, using Philips Photoflux PF1b flashbulbs. They’re quite expensive but I bought 15 bulbs from eBay for about a tenner. They’re good fun to simulate “proper” retro photography.
A note to anyone who is old enough to remember this stuff from first time round: Sorry for being so excited about obsolete technology. I’m just discovering this stuff for myself and I think it’s great! 😀