Think you may end up spending a fortune looking for a large enough vintage enamel sign. You could always buy a large poster and get it framed instead to hang behind the aga? That way you can wipe the glass down easily. Here's a few I liked: http://www.allposters.co.uk/-sp/Tea-Sale-Posters_i13714999_.htm http://www.allposters.co.uk/-sp/Ruf-n-Redy-Lemon-Label-Upland-CA-Posters_i4124132_.htm http://www.allposters.co.uk/-sp/Pomona-California-The-Parrot-Brand-Citrus-Label-Posters_i4292532_.htm http://www.allposters.co.uk/-sp/California-Rising-Sun-Brand-Citrus-Label-Posters_i4292562_.htm Equally, you could just frame a large section of nice wall-paper if you'd prefer something a little more neutral.

You could look at Clarke & Clarke Waldorf in Champagne https://clarke-clarke.com/product/waldorf-champagne/ https://www.roman-blinds-direct.co.uk/product/clarke--clarke-waldorf-champagne-roman-blind

I love natural stone walls so I would leave it as it is. Natural stone walls are always finishing with larger top stones. Yorkshire is full of them! As already said, imagine how filthy the wall would get if it was rendered. I think the abundance of planting definitely needs thinning out. It must make the house very dark. I’m not saying chop it all down, just thin out the number of shrubs/trees. Here are a couple of examples of stone walls in thr country with larger top stones.

Johnathan’s idea of a ‘free standing’ mirror (attach the top to the wall to stop it falling over) in the dark wall area between the windows would be fab. Remember framed prints with glass will also reflect light from the side and give you something to view. I would also consider making the curtain rail much wider to get the material away from the edges of the windows when open. You can see on the floor the shading caused and this would allow more natural light in. The ceiling height and window size mean blinds would not be a good alternative as they would block light and pull the ceiling lower. Also a large cream floor rug will help reflect light upwards. Love the room and with you on the ‘no paint’ option.