Weekend visit 25-27 Feb to de-winterize and prepare the boat
Refilled heater circuit with approx 40% antifreeze - total capacity about 12 litres. Note non-return valve to water supply was fitted the wrong way round. Ran the Bubble stove for several hours (~20 hrs). This did take the deep chill off the boat, but it takes a long time to heat the water to just a modest temperature.
To get the boat up to temperature more quickly, it pays to use shoreline power and/or the engine as well as the bubble stove.