Dave Liquorice wrote:
No.4 Insulate underneath and leave tank as is (I assume it's properly
covered to keep evaporation in summer, in and wildlife, out)

I think so, but I should check.

Would tank lagging be sufficient for a tank in a cold loft, without
additional heat flux from the house?

Is the tank freezing going to be a problem? There is little to
constrain the required expansion, it can just expand up the tank.

Why is there all this stuff about leaving under the tank uninsulated then?
Or is the concern pipes freezing? (The pipes are all in clip-on lagging
jackets, although not sure how effective those are).

Our tanks are stood on boards about 18" above the ceiling joists. The
Ceiling is insulated. Not noticed any problems in the "cold" loft.
Remember mains water will still be above 10 C or so even in winter(*)
and there will be some heat coming through the insulation.

I suppose the worst case is going away for Christmas when there turns out to
be a cold snap - heating down low and no water drawn from the tank.

As it happens I have one of these (2ft 80W version):
https://www.dimplex.co.uk/product/ec...ter-thermostat
and I suppose I /could/ mount it inside the tank insulation somehow,
controlled by a socket controller:
https://www.amazon.co.uk/KETOTEK-Tem...dp/B085Q2NDP9/
(many others available)

Not entirely convinced that's a good idea (what if the thermostat jams on?)
Feels like heaters next to foam insulation isn't a great plan, although
maybe rockwool would be ok.

Theo