The Plain English Society's Nightmare.

Out for a stroll at lunchtime I spotted this in Jason Court, a pedestrian allyway off Marylebone Lane.
It's high on the wall of an apparently unoccupied building, & the letters do appear to spell out words.
Most passers-by don't seem to notice it above their heads, so I got some odd looks as I peered up trying to work out what the message was. But as it was starting to rain quite heavily I gave up & headed off thinking a quick Google would reveal all, but no joy.
One part I can read says "You know what I mean."
Well no!