Locks that work

If its daytime work, and you're only leaving the bike to pick up and drop packages, then a good thick armoured cable lock is the answer (not recommended for any other purpose other than courier use, and leaving for only unattended for only a minute or two). Wear it around your waist, if you're slim (and you are), and then have a serious lock for when you're at the pub and leaving the bike unattended for hours.

Interestingly, you've raised a very valid point - the weight of locks.

I shall amend the list and add all the lock weights that I can find, so people can make a choice based on that as an extra variable.

probably shouldn't wear locks around your waist, if you fall they can brake ribs etc or so i've read on another cycling forum