Thanks for the info Scrambler and Rich, thats helpful.
I hesitate to get the Building Department involved mainly because its so anti-social given most village houses have unauthorized structures of one kind or another. I prefer to address issues like this in a more civilised way.
I would also prefer not to start arguing about my rooftop structure, although it should by now be protected as its already registered under the government scheme.
The next step is to get an engineer to look at it. My neighbour has said that if the engineer says theres a problem, he'll install a lintel, which obviously would be good. Its actually pretty easy and cheap to do - I could do it myself if I had to.