I'm going for the Specter achievment (After escaping prison, complete a mission without alerting anyone, and kill less than 5 people) in the first mission (High Overseer Campbell) and stuck right at the begining. How can I knock out or kill the Gentlemen Callers outside Granny Rags place one at a time without alerting the others? Can anyone help?
Tried sleep darts - soon as 1 hit other 2 alerted.
Tried waiting to see if they'd separate (fell asleep lol).
Gonna try chucking bottles etc and see if they wander over one at time.

Any methods used sucessfully would be gratefully received. Thanks.

Chuck a bottle method works, but you still need to time it at nabbing them and be patient.
Other ways to make it easier to use Possession level 1 and possess a near by rat and distract one of the Gentlemen.
He'll follow you while the other 2 stay there, stay just out of his reach and when he gives up and turns his back come out of possession and choke him, go and hide the body.
The other 2 throw a bottle from somewhere hidden.
The 2 Gentlemen will walk around saying " we know your here", even though they never see you.
Keep an eye on em and time it right and nab em 1 at a time.

Tips:
The 2 survivors near by will go on alert if they see you nabbing the Gentlemen.
So knock the near by survivors out first and hide their bodies.
The Gentlemen Callers won't even notice while you do it.

When standing behind the Gentlemen Callers, the one on the left seems to be the week willed one that will chase the possessed rat.

The same possess rat method works on the 2 thugs guarding Griff the trader.

Thanks. I just kept doing it over and over again until finally made it. You're right, it's all about timing.
THEN I had to do it all over again - but that's another story - basically cos I'm an idiot lol.

Many thanks - not torn out all my hair ........ yet

If you are looking to be unseen, it may be better to never visit Granny Rags at all. Not doing so apparently removes a later-game encounter that is very hard to get through without being detected.

Better and easier way.
Wait until the guards are done speaking all their nonsense and soon the guy at the door will ALWAYS face the door. You can choke out the back guard and carry him over to a dumpster.

Blink back upstairs and run down to the front door. Open it from the side and hide in the room behind the little foyer wall as soon as you do. One guy will walk into your room first, IGNORE HIM. The guy in the hallway will take some time to walk down a bit, but before he gets to Granny Rags hall sleep dart/choke him out.

Quickly or slowly (if you have dark vision its easier) go sleep dart the guy in the other room.

Actually, re: the encounter later in the game, you won't really get detected because Granny Rags is blind. Even though she'll "sense" that you're there, it doesn't count against being detected. (I know this because I had to re-do that level this morning to get a different achievement)

That's useful to know. I've just started a new game in the hope of avoiding detection, and I'd read that it's best to avoid Granny Rags altogether - which is unfortunate because she possesses two useful runes, and quite by accident I managed to get a third from her:

Sorry to disappoint you, but that's always there, you can always open the door and take that one

Oh!

I know it gets offered, but I collected it from there TWICE! Thought I'd been very clever indeed!