Thanks for the reply
"£1400 isn't modest in my book."
*Agreed, it's modest compared to a heat pump and solar PV system though
"Plus if used the way you are suggesting you would need a full safety group"
Not familiar with this term, could you expand please
"You may have a few things to resolve"
Not sure I follow your logic in this paragraph
The cylinder is heated from the heating side of the combi Via a 2 port valve controlled by a cylinder stat. A second 2 port valve valve would be required to allow heating via a room Stat. all this adds complication of course. The hot water side of the combi is not used. The DHW draw is via the the heat exchanger one side being mains fed the other to the taps, as there is very little stored water to be heated in the heat exchanger, way under 15L, so no tundish or discharge pipework required
I do like your preheat cylinder idea, not sure it would help with the long draw off time though