Why is 19deg cold inside now but not last week?

Its colder outside which means the indoors RH is likely to be very lowm cooling you through sweat evaporation

That's about right. In summer 16°C is very nice in my bedroom - today I feel like 21...

I can tolerate 50°C in the desert. I cant tolerate 35°C in a tropical rain forest.

Plus convection draughts even if you cannot feel any air movement

RH is about 62% (it's a 50yr old barometer so no idea how accurate).

Convection is a good shout. Any way to improve the situation? Would it be sensible to run a fan just to ensure the air temperature is mixed up?

I'm afraid the only 'fix' is to add additional insulation to the walls. External insulation plus render also kills the cooling effect of a wet wall plus cold northerly wind. Internal insulation has less of a time lag (while the wall structure adjusts to its required temperature gradient) but requires disruption and redecoration unless you can DIY and you also need to consider the potential for hitting the dew point between insulation and external wall.

I have seen quite a few 60's and 60's houses with cavity walls that have also been externally insulated and re-rendered.

The other thing you feel is the amount of radiated heat you absorb from the room surfaces. With outside temperatures lower, the inside wall surface temp will probably be lower as well. You still radiate at the same rate as before, but now receive less radiation in return.

I notice it when I move from my workshop (well insulated) to the adjacent building (no insulation). Even in milder times of the year when there no heating, and the air temp in both buildings is the same, they feel very different - the insulated place being a much nicer place to be.

You pays your money and you makes your choice... ;-)

(or you keep it in your pocket!)

Just nail some electric blankets to your wall. Heating *and* insulation :-)

Do we wire the cables in series or parallel into the socket?

PS SWMBO is worried about matching the patterns and hanging pictures.