and also to have the immersion heater (permanently isolating the CH
circuit from the HW storage) coming on for 1/2 hour in the morning
and 1/2 at night.

No, you should heat your hot water from the central heating system. It is
much more efficient. However, you MUST ensure that you have a cylinder
thermostat with effective zoning and interlock. This is not just an energy
efficiency issue, but a safety one. Without them, you'll find that the hot
water gets scaldingly hot, up to the output of the boiler, which is likely
to be 82C, rather than the more normal 55-60C.

Christian.