The potable hot water (which would also be used for the bath) and the heating water are never (?) from the same source.

Ah, I wasn't very clear, but it's the same over here. The primary circuit (boiler -> heat exchanger in hot-water tank -> maybe central-heating radiators -> boiler) is a closed system. Potable water lives in the secondary system (water main -> cold-water tank -> hot-water tank with heat exchanger in -> taps).

I don't know much about UK home construction, but in the US, the crawlspace is the area within the foundation underneath the lowest floor joists.

Right, I guessed that's what it was. Most UK homes have suspended (joist-built) floors (the rest have direct-to-earth floors of waterproofed concrete sitting on the soil), but I've never seen one where the "crawlspace" was accessible without serious floorboard-lifting, which I guess is why we don't have a special word for that area. (A common hazard with UK-style construction is accidental sealing of external airbricks built into the underfloor space, leading to moisture buildup and eventually rotting joists.)

I know on the electric ones that there's an immersion coil at the bottom of the tank.

OK, that makes sense if electricity is the primary heat source. UK hot-water tanks, of course, have the immersion coil at the top, as it's not intended ever to heat the whole volume of water.

There's a rather complex diagram at this page (scroll down to "Conventional heating system") that shows the system, even if it's a bit confusing. The "feed and expansion tank" is only there to keep the primary circuit (red/pink) pressurised, and to act as a vent if its pressure rises too high. In normal use there is no flow through the feed-and-expansion tank. The secondary system used for hot-water taps is in yellow; it's fed from the cold-water storage tank, not (as I got wrong above) directly from the water main (light blue).

I'll have to admit that, looking back, it does read funny. But I can't say that it's at all uncommon in speech. Either of your options would be fine, too. Wait. Is it the ``two things'' part that worries you or the ``other than'' part? The ``two things'' part reads odd to me, but the ``other than'' is perfectly common.

It's the "other than". I was going to say something about it looking odd because you couldn't put the reasons themselves there if you hadn't written "two things" (so "This is the same, apart from the location of the heater" sounds OK, but "This is the same, other than the location of the heater" doesn't) -- but I've been staring at it for so long that they both look right to me now. Perhaps I should shut up.

Peter