Checked it out tonight and found that it was the switch in the airing cupboard that had failed on the neutral side.
The circuit is fed by a 16amp MCB. The switched spur that has failed is a Compactum.
Any recommendations for a replacement (thinking MK)
Also it is wired in PVC Flex between the switched /fused outlet and the immersion heater. Is there something better (years ago I would have bought a length of asbestos insulated flex.)