TMC wrote:

adjacent to mine. It is clear that the landlord has partitioned one
of the rooms, half of which is now mine - with a single sheet of
plasterboard which joins across the window. Is such a partition
complying with building regulations?

Sounds very doubtful. Do you mean to say that the new wall butts up
against the glass of the window, so if you opened the window you could
put your head out and round to look in you next door neighbour's room?!

The problem is the noise not the wall

If the wall's constructed wrongly or poorly then it will allow the noise
transmission...

Would it not be better to ask the landlord to deal with the noise or
approach your local EHO to take decibel readings and advise you if
the noise is within legal limits? This they will do covertly

The EHO aren't going to be the slightest bit interested in noise
pollution from another room within the same property though.

If the building is not up to standard then mentioning the EHO to the
Landlord may focus his mind for other reasons than the noise

Agreed!

David