Simon wrote:
Hope someone can send me in the right direction.

I saw a Platform poster on the Northern Line showing what a map of London
would look like in a mirror.

I think it was advertising an exhibition by an artist (Beginning Fr......)

Any help would be great

I worked it out eventually. The idea is to promote the lesser parts of
London, by twinning two different parts on a daily basis. I am not sure
how this is supposed to work in practice.