Big, old... 1/1.5 solid brick walls, high ceilings, vast rooms, a pig to he at.
2010 winter caught a lot of people out. UK is temperate - with exceptions and -23.5oC overnight & -13.5oC daytime w ere that exception resulting in a lot of burst pipes. I recall the -23.5oC temperature reaching out "into" the room despite empty cavity wall, 25mm ce lotex on inside (really not bad for UK), making it feel as though a window was open to it. Very peculiar.
If the place is being vacated, drain down & dehumidifier set to drain somew here.
If the place is "one room being lived in", drain down, dehumidifier set to drain somewhere and heat that key room properly to a MINIMUM of 19oC. Do no t live in a freezing cold damp house, mildew can affect some people quite b adly.
Keeping water moving is critical to prevent if freezing, remember if a floa t ices up even a little there is the risk the overflow can ice and the tank overflows which can really trash a house re black mould and fabric damage.
Colleague had one of these places, used to house 11 accountants it was that big. He fitted a Rinnai gas wall heater from a church (5.7kW or something like that, if anyone remembers them). It heated the whole place acceptably with a dehumidifier to manage the COLOSSAL amount of water in the fabric th at had built up over 1993/94 winter. The difficulty was insulating on the i nside, not the cost, the sheer size of it - it just swallowed 6ft stacks of full size celotex sheets like they were the size of a piece of kitchen rol l.