I studied the DHW thing for ages before deciding what I should do. We had a
standard loft tank and hot water cylinder heated from both the gas boiler
and elec for backup. I installed a pump to get a real shower. Then the
boiler packed up. As did the motorised valve, thermostat and timer. I
studied unvented but after showering at a friends house I realised the
pressure was crap when compared to a pump.
So I installed a combi boiler which gives unlimited hot water to the
kitchen, utility, bath and bathroom taps. I plumbed the heating side of
things just as if I had a conventional boiler and fitted new stat, valve,
etc. I kept the loft tank and DHW cylinder which no only feeds the showers.
Expensive to run? Yes, bloody right BUT it gives me loads of HW and a
proper shower so what's a few extra quid for heating!