I'd be fairly certain the slab was cast in situ, almost a forgotten art now I think, building manholes with bricks. A small local developer was still building manholes with engineering bricks right up to the mid 90's, when I think he packed up, probably due to his team of old hands retiring, and financial offers for his remaining land.
For the bigger chambers the brickwork was taken up from the concrete base, (with 2 courses arched over pipework to bear the load above the pipe), then an asbestos cement/fibre cement sheet was placed on the top to act as bottom shutter for the concrete. Concrete was poured, the sheet was left in place, as it would never rot! Once concrete had gone off then a hole was cut through the sheet to create the access aperture.
Nige, they're supposed to be secret! (Kelvedon Hatch in Essex was, until someone put a ruddy great sign up pointing to 'Secret Nuclear Bunker'.....)
I went to empty a Septic tank once, wasn't familiar with the property so started looking for a suitable cover to lift. (Was a fair size house and grounds.) Couple of manholes later, I found a large square cover complete with handle. Lifted it to find a ladder disappearing into the bowels of the earth! Deciding that clearly wasnt a septic tank I carried on in my search! When the owner appeared later on I asked about the ladder. 'Oh yes', he said. 'Thats my thermo nuclear shelter.'