First, thanks. Richard, Tim, I tried both variants - the whole picture rendered to sepia and the outside left in colour. My choice was 50-50. Second variant is more solid while first adds some "glamour" and make an idea more straightforward, which is not bad too. Depend from the purpose, I think.
I absolutely love it exactly as is. If I were forced to give any suggestions for improvement, the only thing I could say would be to slightly burn in the bright stairs just above and to the right of the window. --Chris