I don't have the number to hand as I'm away... Just to say that it's usually very difficult to get them to answer the telephone. I've tried to call many times, and only got through once or twice. I've also been in there, and the phone just rings and rings...
It's probably not that helpful to say it, but I find just going in is the best way to get mail, if you stand there they will look for your parcel. I just don't leave until they have found it, whatever the queue behind me.
The staff in there are actually OK, and do try to be helpful.
do not waste your time ringing se1/se16 delivery office.
if you go to the office, you can see the staff personally ignoring the 'phone.
often it is permanently engaged and if you ring BT to report a fault, they will tell you the number is fine and never answered; they will happily volunteer this infomation as the number has remained unanswered since 1978 to my knowledge.
oh, btw, don't ring the 0870 number either to rebook or complain, they do the 'push x key' for ages your call being an absolute nuisance for them but very profitable, and then just when you think you will speak to a human, the message tells you they're busy, refers you back to the website from whence you came and promptly hangs up!