In message , Clive
writes

I'll be as clear as I can. I don't know anything about London but a
friend of mine that lives there said "Nine Elms" had now changed it's
name to "New Covent Garden",

I think that may be an estate-agent term. Officially the area is still
known as Nine Elms.

I would like to know what is the history and subsequent progress of
the planning of this new line.

There's a lot of information at http://northernlineextension.com

I know the Kennington loop only connects with the west end side of the
northern line because back in the 60s and 70s I worked on it, will it
stay in service?

Almost certainly, as the proposed terminus of the extension at Battersea
is only two-track, which would not be sufficient for turning back the
full peak-time service of the CX branch. I don't think any service
pattern has yet been proposed, but I guess that it might well involve
alternates trains turning at Kennington and Battersea.

The four proposed routes (of which number 2 is the preferred option) can
be seen in:

http://northernlineextension.com/med...ns_lr_pages.pd
f

You will see that they all involve the extension turning off one side of
the existing Kennington loop and returning to the opposite side of the
loop.
--
Paul Terry