Just wanted to jump on here as I began a 9-square-meter fairly modest extension to an 1840 lower ground floor flat in central London. Some nerve wracking moments while opening up and supporting the external, load bearing, wall but I was lucky enough to have an experienced builder (has worked on and restored G2 building in central London), structural engineer and approved inspector. Before I began works and I was still at the planning stage I came on here to get a recommendation for someone to make a timber 3/3 sash window for me. A member here recommended Firth in Stockbridge, Sheffield. Paul was brilliant and dropped off the window to me in London. Highly recommend him!