I saved the life of one such poorly positioned cyclist at Oxford Cricus just two weeks ago - standing waiting to cross south to north when a bendy bus was coming past, signalling to turn left into Regent Street - idiot cyclist, talking on mobile phone, comes up behind the bus and, ignoring the indicator, starts to overtake on it's nearside as it slows down for the corner. Seeing what's happening I shout "Stop - Its fucking turning!!!" and he slams on his brakes and stops about five yards short of the apex of the curve. The bus driver, who had double checked mirrors saw him and stopped too. Prick cyclist then cycles past the now stationary bus and blithely carries on down Oxford Street, shouting "Use your fucking mirrors" at the driver as he does so ...
... wish I'd never bothered, to be honest.
(And he was one of those cyclists dressed in office clothes, with no helmet, not some professional courier)