What sort of cost am I looking at for boiler, heat store, controllers?

For good quality components, you're talking between 1500-2000, including all
controls, zone valves, condensing boiler, heat bank, programmer,
programmable room stat. It could be done cheaper, if you intend to use less
regarded brands, however the above will get you something decent, like a
Worcester Bosch Greenstar and DPS heatbank.

Basically a stored energy mains pressure hot water system will cost you
twice that of an equivalent quality instantaneous combi in parts, although
the time spent fitting will have less of a difference and hot water
performance will be in a different league.

Christian.