A thermal store will slot in easily as long as the CH is run off a coil in the thermal store. Unless he wants to run the CH off the cylinder he hasn't much to gain with a thermal store only extra expense.
He hasn't mentioned the mains pressure, but a thermal store can still run off a tank.
He hasn't a thermal store he has an unvented cylinder that can be safety made vented without too much effort.
either its an open vented cylinder or its an unvented, if its unvented then it falls into the G3 remit of the building regulations, as such is work which can only be undertaken by a G3 qualified engineer.
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