On 30 Dec 2020 16:08:10 +0000 (GMT), Theo wrote:

I suppose the options a

- add a better jacket to the tank
- add a heater on a frost stat inside the tank
- add a heater on a frost stat to the side/bottom of the tank
- something else?

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

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.

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.

We have had a couple of pipe bursts due to freezing. Same pipe, in
the garage with no heat and only when its been down to -10 C ish for
several days.

(*) PL footy grounds use this "warm" mains water to thaw the pitch
surface on days when it's close to or just below freezing.

--
Cheers
Dave.