Thanks for all your replies so far - really appreciate it. To answer a few of the questions that have come up (and ask some of my own):
@Pleasehelpmedress What sort of ceiling lights do you think would work best, if I went for two like you suggested? Do you think long pendant ones of some kind would work? (The ones there now seem too close to the ceiling for the height of the room, which is 280cm.)
I have no plan to change the floor (though I think a large rug would be good somewhere depending on the layout - that's another choice I need to make!) As far as style goes, I like to keep things minimal in terms of "stuff", but I like colour and cosiness where it makes sense. It is quite a dark room though, being north-facing, so "bright/airy and cosy" would be ideal. I'd like to keep things correct for the period (or at least, not looking out of place), so that will influence choices.
I'm not currently planning to put in built-in shelves, because I find them to be clutter/dust-magnets and they're currently not needed (and I don't want to encourage accumulation of clutter :D). What's your reasoning for not painting the fireplace wall a different colour? I don't think I have a good sense for these things.
@trickyex I'm hoping to use the room mainly for social and musical activities (and some of my social activities are also musical). Re: the piano, this one is "hybrid" (a type of electric that has hammers to mimic the feel of an acoustic), which is why it has that weird shape that looks like a baby grand from the front. This is going to be replaced with an acoustic at some point in the next 6 months or so, but I'm probably going to go for an upright. I agree a baby grand could look lovely in the bay. When you say "facing in", what orientation are you thinking of exactly? If I get an upright, do you think the current wall is the best one?
@MadMadamMum Don't worry, I will be keeping the parquet! I think your point about keeping it light above the picture rail makes sense. Re: sage green, and greens in general, I've tried but I just can't make myself love it (but I do think the room in your first picture looks lovely and cosy, with the richer colour). I like many shades of blue though - any thoughts in that direction?
@MidYearDiary Re: the floor-to-ceiling glass, how much of the bay walls would you knock out to do that (just the bits on the same plane as the door, or the diagonal bits too)? Would you worry about it looking wrong for the period like some other posters have mentioned?
What's the reasoning behind removing ceiling lights completely? How would it work to wire them altogether - would it mean lots of wires visibly connecting them around the room?
Re: colour, I'm really torn on the light vs. dark thing. I love deep jewel colours in general, and I've seen (mostly in pictures) rooms that look stunning in deep cosy colours lit by artificial lighting. But I don't know if it would feel too gloomy during the day. I also don't know if I'm more likely to be using this during the day or evening, as my life isn't really in a constant phase at the moment. I guess social things are more likely to happen in the evening, but music could happen at any time (when it's not a work day). Do you think there's any kind of blue that would work in the room, given your points about cool tones? Thanks for the links - I'll take a look.
@Franklet Thanks for the link! I'd like it to look in-keeping with the period, which I guess is quite elegant, but I'm not keen on matching furniture (I prefer complementary). I also quite like the idea of mostly chairs of various kinds rather than sofas, for more configurability, so I don't know if I'd want two sofas facing each other. Having said that, if I saw a lovely example that would work in the room, maybe I would change my mind. It faces north (with a bit of evening sun coming in the diagonal NW window in summer).
@AudiobookListener I feel like I was always advised against brilliant white, but I'm not sure exactly why. Would you paint the fire surround in that?
The windows are already uPVC (and very old ones at that!). I had a window guy around who seemed very keen on aluminium frames, but I'm not sure if that would work here. I'm wondering if thinner uPVC would be the way to go, with fewer of the "dividers" between panes of glass, so there's a bigger window surface area?
@Seaside3 Do you know if there's anywhere I can find info on what makes the bay correct for the period? One thing I've never been sure of is if the bay is original or an extension (the house next door which has an almost identical layout doesn't have one), but I don't see any obvious signs of it being an add-on. I definitely don't want to ruin the proportions or charm by removing things I shouldn't.
Re: radiators, the pipes come through the walls (not the floors) in opposite corners - one in the corner to the right of the fireplace, and the other in the corner near the piano. This means the radiator can go anywhere, but if it's far away from either corner there might be some pipework that needs boxing in. Why would you put it along that wall rather than the back wall? Where would you put the piano in that scenario?
@mathanxiety Definitely!