I may also have just seen that B aswell, on the A12 south heading towards the Blackwall Tunnel, somewhere near the Leyton turn-off, about 15.10! It was definitely those colours, with a third brakelight on the boot, and a bootrack aswell. Actually having looked at that photo again, is that a bootrack or some kind of double spoiler thing going on?
In a way I hope it's not anyone from the forum - he'll be paranoid soon, everyone logging where he drives!
JUST REMEMBER, SILVER B OWNER, PROBABLY FROM SOUTH LONDON: WHEREVER YOU GO, THE FORUM IS WATCHING!!
PS If it was you, I was in the much scruffier green N-reg B that was hammering it down the outside lane because I was late for an appointment