I've just had a friend from teh city ring me up and ask me to look out of my windows to see what direction a very large plume of smoke/cloud is coming from and I can now see what he means.
Also as he was ringing we've had a number of fire engines and other flashy-light/siren-wailing vehicles rushing up the road in it's general direction.
I haven't heard any explosions so I'm guessing it's just a frie but it looks like a fairly big one. From my house it's in the Finchely central/Hendon/Brent Cross direction (West South West from me).
EDIT: Here's the BBC News Report of the fire. Glad to know it wasn't anything really major...