In reply to simon clarke:
dunno - but I got flashed on the way in to London on the A4 today and my speedo said I was doing the speed limit (40) - will be seriously miffed if I get a ticket!
and yes, there's a cpl on the A40 coming into London and 1 on the way out (a few hundred meters after you come off the westway flyover) - generally I think the speed is just to do with the volume of traffic.