FWIW at T5 flights "close" for commercials at around STD - 45. Normally it is only then that the allocation of standbys to seats starts. Sometimes if you're on an fairly empty flight you may get assigned a seat before the flight closes but if it's pretty full you can expect to "go to the wire" and end up sweating it out in the staff travel waiting room. Having got a seat (by way of the desk previously described by Sid) you must make sure you are through "conformance" ( the gates before security where your boarding card is electronically read to give you onwards access) no later than 35 minutes prior to STD. If staff travel are struggling to cope with numbers they may be able to reduce the 35 minutes ( it has certainly happened to me) but don't bet on it.
To sum it up.......be prepared to run, both from staff travel to conformance and again possibly to the gate after security.....good luck.
( if you have the option of putting your mobile number in the booking it's worth doing. If you are fortunate and seats are released early you'll get a text telling you that has happened, might save you hanging around land side when you don't need to....)
Last edited by wiggy; 23rd Oct 2015 at 08:28.