Thanks, I rather hope NMEA does come on, I've read their (his/her?) posts and they seem very knowledgeable. I don't understand a 'pressure jet boiler' but I've have a google and see.
The heat exchanger is https://www.butlertechnik.com/installation-accessories-c63/webasto-motor-home-plate-heat-exchanger-with-mixer-valve-4111209a-p1516 - I have one on the sail boat. It's fabulous, within ten minutes (top c) you get unlimited amounts of hot water, bit like a combi boiler. Obviously the tank water is also getting heated.