It's very likely, in a 120 years old building, that the drainage is combined. In relatively modern construction the foul and surface water drains are separated, with the foul water going off to a treatment works and the surface water going into a nearby watercourse or soakaway. I think the subsoil in Bedford is predominantly clay.
Personally, if it's possible to install a drainage gully at the lowest point and connect a 100mm drainage pipe into an adjacent manhole, I'd quietly go ahead and do it.
Personally, if it's possible to install a drainage gully at the lowest point and connect a 100mm drainage pipe into an adjacent manhole, I'd quietly go ahead and do it.