Timothy Baldwin wrote:
In message , Helen Deborah Vecht
wrote:

The nearest Underground entrance to the mainline platforms has stairs
and is for ticket holders only IIRC.

Not any longer, the ticket office is back down there now.


It might be worth summing up various strands of the route advice in one
post.

The original poster will be arriving at Kings Cross mainline station.
He/she can take the new lift adjacent to the underground entrance
*outside* the station at street level, down to the underground station
concourse. From there, go along the passage to the metropolitan /
circle line concourse and take a second lift, down to the platforms.
Then take a westbound circle line train to High St Kensington and
change to a district line train bound for Wimbledon, which will come in
on the same platform.

Finally you've just got the steps at Southfields to negotiate.