Who blesses themself after sneezing?

Did you say "thank you" to yourself? If not, you need to be more polite.

If you live in a sane country you don’t have to let your landlord in. And they have no right to control who you have over.

What century is this that the landlord thinks it’s their business if a renter has a visitor over? OP I am sorry you are living in some time warp that put you back in the early 1900’s. It’s one thing to require people who might be residing there to be on the lease, but this is pushing the ridiculous.

Where I live landlords need to give 3 days notice before entering without probable cause of contract violation. I don't think a day guest counts...

First thing I would take care of is those paper thin walls, or find out where they planted the bugs. Who know what else they are hearing.....or seeing.

What sort of gestapo landlord is that??

It really depends on the situation. Depending upon the agreement, the landlord may have the right to inspect the renter's apartment. If the renter is renting an illegal accessory apartment (i.e. someone's basement), the only recourse the renter has is to find somewhere else to live, before reporting the illegal apartment.

in the UK I believe they need to give 24hour notice if they are coming round

Well you can tell your landlord, "Bless your little heart for checking in."