Im thinking of installing a 22 or 24 litre per minute unit that is mountable
on the wall outside - I prefer these smaller boxes to the storage units
because they take up too much space (and I hear the continuous are cheaper
to run long term)

I have been offered one (a bosch? cant remember) with an electronically
controlled thermostat which I dont think I need, least of all I dont want to
pay to have installed.

Can anyone tell me the ins and outs of these continuous units and whether I
need know anything particular? And what they might cost to install? (I know
they are more expensive than the storage type)

thanks.