"He then pulled the wires out of the plate that fed to the star wired extensions.
This bit confused me. I thought some testing was done in the test socket so no extensions should have been connected but the whistle was still present.
I also am confused - I assumed the "TEST" socket behind the face plate isolated all extension wiring..
The previous BT engineer who fitted the master socket opened a grey wall box containing a multitude of punched in cables.One pair carried the phone line into the box and the rest were presumable the termination points of all the extensions in the flat - star wired from this box.
He released the in-coming pair and fed it into the new master socket then fed a pair back out to the box where the original pair went.
At this point nothing was fixed.
The last engineer replaced the face plate with an ADSL splitter plate. Then he pulled the wires off that went to the grey box and issue solved. In replacing those wires the polarity was reversed and problem solved.
I must assume that somehow the test socket behind the plate was not disconnected from the internal wiring - but I am only guessing as I do not fully understand how all this works.
So again - fixed (somehow) and thanks to all