Thanks for the reply, really appreciate it.
As far as I'm aware the window has always been there, and has always opened that way. The maisonette we live in used to be one house, but we separated into two and the downstairs converted for an older lady.
It's an open fire. From what I can work out, our bedroom used to be the living room, with the bigger of the two fires (which is now been boarded up with an air vent stuck in) and the one in the now living room (where the window opens onto the flue) used to be a dining room.
From what I can work out the boiler is a Potterton profile, though it looks like some ID stickers or what not have been removed.
Does the window when open count as a opposite surface?