Robert Inder wrote:

http://www.oldhifi.com/tannoymonitorgold.jpg

The socket for the plug I'm talking about can be seen on the chassis
of the speaker, about 80% of the way down the centre line of the photo.

Can anyone tell me what a plug/socket of this type is called? And
where I might be able to get a replacement plug?

I have a vague recollection of them but can't remember a name.

They're extinct now ! You'll have to replace both the socket and plug. A good
thing since it's was lousy choice for the job in the first place.

Graham