I have solved my own problem - the solution may be of interest to others who have the colirifier in the stern.
The background to this is that I discovered that when running off the water from the main tank, the colirifier remains full and so may suffer damage in severve cold conditions. This is because as with home hot water tanks, water is extracted from the tank by forcing cold water in at the bottom.
A solution is to unscrew both hot and cold pipes from the taps in the galley; place the cold water pipe in a bowl and then blow down the hot supply pipe so reversing the process. Within few seconds water will start to flow out of the cold pipe as a syphon process is started. This will completely empty the colirifier.
Don Harvey
Don Harvey