Thank you for your advice, however a good friend of mine has stepped into the breach and dismantled the bit that was leaking. A diaphragm was removed and appears to be intact, so we have scraped off some old sealant as this seemed to be preventing a water-tight fit, and it seems to have fixed it. I am going to the boat tomorrow and will run the taps and check again.
In due course, I may well have to replace the heater, but for now . . . . . . . . . . it seems as though I have been very fortunate.
Thank you so much to everyone who has given me the benefit of their knowledge, expertise and opinion, I am exceedingly grateful.