Thanks for the answers.
The current 800w heater, with the option to go to 1600w when hosting guests, has worked fine for 17 years, and never left us short of hot water. windtalker's suggestion to go to 3kw would mean us sitting in the dark regularly as the ICP would be forever tripping out, so wont work for us, sorry to say. (we also will not be upping our incoming power supply).
We had previously discounted Kavanagh's suggestion as we did not want to be a slave to the gas bottle man. But it has made us relook at the possibilties and we discover we have mains gas outside the building, although no one in the building uses gas. I wonder what the logistics and cost to have us connected is, and will certainly investigate.
Meanwhile if anybody does know of a sub 1kw heater available on the market I would be obliged.
Mac