Really old Webasto problem.

On my boat I have I believe an original Webasto circa 1978. It has the clockwork control mechanism. It works really well at full heat, but when I go to half heat it cuts out. The manuals available seem to be all about the new models so I have no idea of the mechanism that takes this heater to half heat.
Does anyone have experience of this problem?

I think these burners manage duty cycle by turning on and off. It may be working as designed

I think that gholz is right. If your Webasto is similar to my Esbar of similar vintage it appears to work by cycling the pump on and off, but I've never gone below to listen to the pump. My "clockwork control mechanism" is a rotary switch. The options are off, air only, low heat, and high heat. There are contacts that turn on the fan and different ones that are for the two heat settings. You might have some dirty contacts on the switch, or a broken wire. I'm not sure what controls the cycling at the furnace end. It could be a set of relays or maybe a direct connection to the pump. The Esbar site has both manuals and troubleshooting guides for their units. You might be able to get an idea of how your unit works by looking at them for similarities.

On some models "half' is created by a resistor on the rotary switch simply reducing the voltage to the pump motor on the burner.

Check the voltage coming out of the rotary switch.

Webasto heater

Thanks for your comments.
I have actually been able to get the handbook from Webasto. They had one in their museum!
The local agent did not have a manual and was unable to comment on an item so old. He said that when he was trained they were not taught about the old HL series particularly with the clockwork starting panel. I have seen many boats that still use these systems.
Webasto said that the only spares that you can get are the glow plug which are presumably the same for many heaters. They also said that my particular problem was probably the resistor which feeds the reduced heat circuit. I don't know the logic for that circuit yet. The motor certainly slows down and therefore the fuel pump must slow down also. I'll have to see if I can measure some voltages next time that I am on board.

ON most units the air delivery is not slowed down , only the fuel delivery pump / motor.