What a load of nonsense, this Freddy guy is an amateur. Hidden void behind a door in his library?
In my house even the library itself is in an unknown location and has never been seen by human eyes.
... ... So he found a service void with some pretty modern looking pipework, an underfloor space with sprayed on insulation...
I noticed that too. Perhaps it was sprayed on in the 19th C to use some government insulation grant back then