Thanks for the reply, Erwin.
I finally got around to changing this bulb today. The connector is just like a rubber grommet in the headlight, so a gentle pull is all that is required to remove from the back of the light. I removed the headlight from the bike as I didn't want to risk breaking anything.
The bulb is a wedge type bulb. There were markings on the bulb, but they came off when I removed the bulb from the fitting

The bulb just pulls out of the rubber socket. I used a standard North American bulb type #168, which is rated for 5 watts. Euro spec is a W5W.
It seems brighter than it used to be (before it burnt out

). It could be that the original one was 3 watts, but I don't know for sure.
Oh yes, I also put dielectric grease on the socket contacts and a bit around the grommet fitting to help with moisture.
H.