Paul Corfield wrote:

My brain now hurts but please take into account comments from others -
especially about the National Rail extension ticket options and
availability as I think that is key to giving you an answer even if it
is a bit of a bother to buy.

Ok, I shall attempt to buy one of these NR extension tickets from New
Cross Gate station one afternoon soon, and report back here what
happens. Cheers.
--
Andy Roberts