After some advice.
I moved into a Victorian terraced house about a year ago, which used to be a shop (20+ years ago) so has a bresummer beam across the old shop front. The upstairs windows lean inwards, suggesting the beam is sagging, so I took some of the porch apart to take a proper look - Google street view suggests no significant movement in last 6 years.
Appears that the old timber beam had woodworm, and at some point they put a new timber beam in underneath! Seems a bit strange they left the old one in there?? I would like to get a professional to take a look but I'm not sure who would be best - timber specialist, structural engineer? Don't want someone to try selling me unnecessary work, but also don't want the house to fall down!!