Anyway it turns out to be ancient New World gas fire / back boiler type affair. From the description of the fault it initially sounded as if the pump might be at fault, since the gravity circulated hot water was fine I was told. However, I could not actually find the pump! Looked in all the normal places. Do these fires / boilers have a integral pump system boiler style, or had the installer been really creative with its location?
 The reality was somewhat different - in fact the main burner was not lighting at all, and there were no volts getting to to the main gas valve.
The hot water was "working" since there was a tank full of it! The pump was also working in that you could hear water flow and feel vibration from it when the stat was calling for heat.
The fault turned out to be a loose mains inlet plug - the connector for which was mounted on the end of a bit of flat wiring cable by the look of it, and it had simply sprung out of its socket due to the stiffness of the wire. Returning it to the socket restored normal operation.
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