Thank you @Ferdinand @Andehh and @IGP and I'm sorry for the very late reply to all your helpful advice!
I hope I have answered all your questions/points below:
I am pretty sure that I have established that there is only a very small cavity on the wall where the second skin has been added (seen from back garden pic) which wouldn't be enough for any cavity wall insulation based on your advice. The other walls don't seem thick enough for there to be a cavity, or again only a very small one.
I applied for a government scheme some time ago where they send somebody out to assess for wall/loft insulation and then assess how much you might need to contribute financially but am still waiting to hear back. From memory, the criteria was council tax band a-c and EPC rating D or below.
I am too short even on the top rung of our ladders to see in the loft (haha) but am told there is some insulation, however, not as much as there could be. I was holding out for the gov. scheme but I applied in April and am still waiting for an assessment.
Mine appears to be the only one rendered, any ideas what it could be hiding? Am I being naive in hoping it was just for decorative reasons?
We have now ordered some secondary glazing with magnets to fit to our one original window and it should be with us in a couple of weeks.
I believe our floors downstairs are solid. I know very little about floors though so not entirely sure what this means for us. We have a beautiful original Minton tiled hallway (as do many around here in Stoke, as it's where they came from) which I would hate to remove. We also have a nice parquet floor in our dining space (other half of the room is the living space and is only carpeted). So really I'm not sure anything could be done flooring wise.
We have a log burner in the living/dining room but it only heats the one room. We can get the room nice and warm but by the morning, it can be as cold as 8° again when it's frosty outside.