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@BrickMan @Mr_Smyth - thank you v much - super helpful. I have an older (2013) unvented gledhill. Unfortunately when it was fitted the link to the back boiler on the Rayburn was disconnected.
I think what I want is:
1) remove electric shower and plumb in real shower
2) replace cylinder with unvented with off peak thermostat element - if you run out of hot water, light a fire
3) get an interface that allows the Rayburn back boiler to provide heat to the unvented cylinder, and give me one water-filled heat release radiator.
4) replace the ancient storage heaters on the economy 7 probably with infra red panels, and run them for a bit at the start and end of the night when it is cold
5) get economy 7 circuit extended upstairs to do the same
6) insulate
7) cry in bankrupt
t.o.-
Or get quantum storage heaters, they retain heat better than old style heaters and are computer controlled so learn how much to charge up based on ambient temps. You can tell them if you are in or out and program the times to suit you. New ones run on an app (I have the older version) don’t think you can even buy heaters without a proper timer now and you will not get a C grade on EPC
Mr_Smyth
Yes, got a Gledhill stainless cyclinder here with direct heating, which means a threaded hole for a single or double immersion, they only loose around 1kwh worth of heat during 24 hours (depending on room temp, water temp, science etc) which is very low, literally a waste of time to try and put another insulated layer over it as it is already a very efficient system.
But you can get them with a single or a double coil, or indirect, which means you can heat the water with a boiler on a separate circuit, or using solar pv type panels (lots of new builds use this method so you get what you get from the solar setup, then use the immersions if needed to heat the water the rest of the way).
Using nighttime rates works best with these as they are so well insulated the 0.5 to 1.0 kwh you could possibly loose at the worst case scenario due to loss is outweighed by the cheaper energy rate.
needs to be installed by someone competant, there is a 'G3' requirement. We got a plumber from over 2 hours away to install ours, but I actually threw him off the job as he literally had no idea what he was doing, spent best part of 3 hours swearing about regulations and why his way was better. Insurance assessors would likely be able to spot the 'imaginative' plumbing solution he had dream't up to provide the D1/D2 pipe a waste route. Ended up doing it myself, not so bad if you are competent and dealt with plumbing at least once in your life before. But would recommend an actual qualified plumber, least it maintains a longer warranty for the cylinder.