Oh yes, and one more thing!
I've just looked at CAP 413 and while it doesn't shed any more light on this matter I would like to raise your collective awareness to item 1.2.3 under Transmitting Technique where it clearly states:
1.2.3 After a call has been made, a period of at least 10 seconds should elapse before a second call is made. This should eliminate unnecessary transmissions while the receiving station is getting ready to reply to the initial call
So next time you check into a busy London frequency, please wait 10 seconds to allow us sufficient time!
(Seriously though, one of THE most frustrating thing is when people jump straight onto the frequency after QSY without first taking just one moment to see if we're waiting for a response. PLEASE listen first and transmit second)
Sorry I digress....