Andy Hall wrote:

I'm trying to find a hot water system that will fit into a
cabinet whose outside dimensions are 500mm wide, by about
2m high, 600mm deep.

[ ... ]

I think to fill a reasonable size bath I'll need about 150L
capacity.


You can get them made to size, but unfortunately you can't fit a quart
into a pint pot.....


In case that wasn't clear enough... let's do some scary multiplication,
in scary units called "decimeters". There's 10 dm in one m (hence 10cm
and 100mm in one dm). And a cubic dm is (to all reasonable purposes) a
litre. So the outside space here is 5 x 6 x 20 dm, giving a total cubic
capacity of 600dm or 600l - so the tank'd have to take a quarter of the
gross volume, more like a third of the internal volume once you've got
cupboard doors, sides, and (oh) access space for the boiler.

If you get a custom-made rectangular tank, the biggest cross-section it
could reasonably take would be say 4dm wide x 5dm deep, leaving sthg
under 10cm of access space for pipework around two of its four sides,
and covering it with superhypercelotex to get maximum insulation for
minimum thickness. To get your 150l with a cross-section of 20 sq dm,
you need a height of 7.5 dm - more like 9-10 dm once you allow for more
realistic access around it. That's half your height spent on the tank.
Can you fit the boiler into the remaining 1m of height? Sounds at best
expensive, cramped, and hot.

I think you'll have to settle for a combi, or find space for a tank
somewhere else!