During the work I noticed that an old mural underneath was exposed:
I happen to have some photos of the old mural when it was first painted in about 1998. As I recall the old corridor had lots of unofficial graffiti and the rail company approved a brand new mural. I assume that they didn't notice for a while that it featured the famous drug dealer Howard Marks - or at least that's who I assumed it was (check the photo).
On the other side of the corridor there was a reference to South London rapper Roots Manuva with the words 'The Prophets rise again... the wise will one day rise'. Not sure who the bloke with a beard was supposed to be.