Harry Bloomfield, Esq. wrote:
Insulating will help keep the heat in, but only for a while. I would
suggest nailing carpet around the frame of the support stand and
leaving the area over that ceiling uninsulated.

I doubt carpet would help much, but celotex as suggested would.

Another way, would be to add a source of heat to the tank. Can a cheap
test tube type heater, with a built in stat, be set to operate at 5C?

That was my thought: don't waste heat for the 99% of the time it's not at
risk of freezing - and also avoid transferring heat from the hot loft to the
house on a summer's day.

There are things like this:
https://www.tankblanket.co.uk/why-tankblanket/
but that's for motorhome (12V/8.5A) use. There doesn't seem to be an
equivalent for domestic use that I can find.

I could drop an immersion in, but I can't find a suitable frost stat that
will measure the temperature of the water (not the air temperature).

The only other kind I can find are fish tank heaters, which don't go cold
enough. 'Pond ice heaters' may do but it's not clear they have thermostats,
rather than things you just put in on cold days.

Theo