Well, did this job today, as assistant to local to my Mum's house semi-
retired plumber.

What I'd described as a tray was actually a sheet of lead laid over a
frame, with a drain hole and a lead overflow pipe soldered into it.
Frustrating that it had leaked, but couldn't see where.

The old tank just looked horrendous inside, with massive and thick
"growths" on the walls. There were at least a couple of rusty spots on
the outside, where water had been seeping through.

Apart from the drain for the tray and the tank overflow which were
both lead, the plumber reckoned the other pipework was very heavy duty
copper.

He'd turned up with a big tank, but it was an inch out from going
through the loft hatch. This strikes me as daft - many hatches are
probably nominally 2ft square but in reality end up a bit less. Why
not make the tanks an inch or two smaller so you'd be sure they'll go
in? Then you wouldn't have to faff about with 2 tanks.

He had a heck of a job getting rid of the airlocks from the domestic
hot water - that would probably have defeated me without his range of
various tubes.