In article , The
Medway Handyman writes
Ron Lowe wrote:

Option 1b) Small immersion-based pressurised cylinders under the sink.
These work well. They store a few litres ( and even more pints! )of
HW under the sink, in a smallish unit. So long as you don't want to
fill a bath from it, it's good ( hand-washing, etc. ) Advantages: Easy
retrofit assuming cold supply is adequate; very close proximity:
instant HW. Disadvantages: Lowish volume of stored HW.

Are those the ones that require a vented hot tap Ron? If so the OP should
consider the hige cost of the taps.

Some do, some don't, the better ones are pressure rated and say there's
enough expansion capacity in a small run of pipework to do the job but
if not then a tiny expansion vessel will do. On these there's no need
for the expensive taps, otherwise ouch.

TLC have some good examples.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs