On Saturday, 27 January 2018 17:51:10 UTC, George Miles wrote:
But I've removed the loft above the bathroom: now the tank's at the
top of a very tall room and over it is an OSB3 ceiling, 8 inches
of kingspan , osb3 and an EDPM rubber roof.

Then it's in the warmest part of the room and as long as you heat your bathroom it's not going to freeze.

Owain