If you are opening the whole space, I think that’s a mistake. It nice to have a room separate from an open plan area. I would block up the door from the sitting to the dining. Put the kitchen on that side and you could have a better run of units. You appear to be knocking a lot of walls but maybe that is just the drawing? The bathroom is in an awkward spot. Better to have on right under stairs if possible. A built in window seat would work well under the window in the sitting room and rebalance the off centre fireplace.

Thanks for your ideas, Siobhan.

So one of the reasons we want to open up the space is because the back of the house faces due south which results in the living room being pretty dark (despite large window). The space is also not particularly big and we do prefer more open plan living. There would be an RSJ running the width of the house so we could always add doors or a stud wall down the line if we felt we needed it. Our plan for now is to use screening to separate the two areas while keeping them open to one another (floor to ceiling shelves or vertical slats for example).

The space with a "B" you see in the original floor plan is actually a small room that used to house the (gigantic) old boiler, but is now a large void in the centre of the house. This will be removed. It's load bearing, hence the RSJ. I can't really think of a kitchen configuration that doesn't need to use some of that space (what you see in the drawing is a fridge and pantry beside one another surrounded by a stud wall). Removing it, even partially, enables us to steal back some of the void for the living room.

The living itself room is a nice size and width, but you're right, the off centre fireplace does make it slightly awkward to lay it out in a way that makes sense. The bay window is almost full height and beneath it is a low radiator, so no space for built in seating unfortunately. Our current thought here is to have a corner couch along the hall wall and coming down in front of the winow. Then have a console table behind the couch below the window, plants etc. We've tried it and it makes the room feel a lot better, but you then lose that extra space by the window. Alternative is just to have two 3 seater couches, one along the hall wall, and one under the window. Decisions, decisions :D

Plan is indeed to have a WC under the stairs (at some point down the line).