Separately, we're aware that heat flows up the stairs, cooling the living rooms (which we'd like warm), and warming the bedrooms (which we'd like cooler).
Longer term, we're considering insulating the walls (the back wall externally, and the front wall internally), but that's a way off, but there are some changes we've been able to make immediately. Firstly, the long-unused curtain rail separating the landing has been put back into use over the last few weeks. Having proved workable, and having made a difference to the landing temperature, I've replaced the track with a pole, which has meant a set of tab-top curtains that were spare have replaced the unattractive curtain that moved there from my dressing room. The curtain now splits the landing into a small area at the top of the stairs and outside the two spare bedrooms, and a longer landing with the master bedroom and bathroom off it. This means that heat from downstairs only really escapes into the new small landing space, and not into the bigger rooms (which also have loft hatches).
I've also put a thick curtain up across the back door. It'll mean access through that door is slightly complicated, and that we won't, during cold weather, go out for any length of time using that door, but it should really help with the drafts.
This obviously doesn't work so well with the front door: a normal curtain across that door couldn't be drawn behind us on leaving, meaning it wouldn't be in place during the day, or if we go away. Instead, I have constructed an ingenious contraption that works to open and close the curtain with the door, while entirely covering it (that is, extending beyond the door's recess on all sides. Photo when I can. Basically, there's a shelf bracket and short vertical pole mounted at the top of the handle edge of the door, with a curtain pole 'cup' at the top. This brings the cup forward out of the door recess, and up above it. The cup can rotate, and the pole can move laterally along it: both of these are necessary freedoms, because of the non-coinciding pivot points of door and curtain pole. On the hinge side, on the wall, the curtain pole's mounted on a bracket modified to include a vertical bolt, through a hole in the pole. This is the pivot of the curtain. When the door's opened, the handle side of the pole moves with the door, rotating around the hinge-side pivot: it also moves hinge-wards, in the door frame of reference, and ceases to be parallel to the door (hence the requirements for sliding and rotating the handle-side fixing).
Relatively simple to make: you can buy portiere curtain poles, but those for recessed doors are expensive (~£100-120), and all this cost was £30, for a curtain pole and a shelf bracket, and a few bits and pieces (scrap wood, to mount things, and a machine bolt for the bearing).
I then put up similar affairs on the doors to the two spare bedrooms, to help compartmentalize the landing. Simpler, because there's no recess; instead of a bracket, the handle-side cup is mounted on a small vertical plank, which means the curtain pole is above the door level. The cup through which the pole slides needs modifying, though, because of the change in angle between door and pole; that's easy enough, though.
Lastly, I've put up the new outside light above the drive (much nicer than the old bulkhead lights), and wired in a PIR sensor so the lights come on when you approach the front door. Needs some tweaking (so it comes on for longer, and doesn't trip until you're closer), but very pleasing.