Thanks for all your replies a picture does paint a thousand words
so here we go;
The building on the right is mine and the neighbours is obviously left, the small extension to the back was built many years ago and is my utility. The neighbours extension is built on the boundary so it would be a party wall.
My initial thought was to basically extend the small pitched roof at the back all the way forwards, allowing it to be fixed to my building, my small extension and to the neighbours property.
However a vent, maybe flue, from the neighbours side would be in the way so it would need to be lower but I thought following the pitch would be more aesthetically pleasing. There are also many guttering overhangs on their property to mine, and they leak frequently!