First of all, you have my sympathy for the situation you're in, noise nuisance is a very serious issue (and one which I've had to deal with in the past) and
is one which is taken seriously by Council ASB departments.
The operating hours of the local noise team sound fairly standard, bear in mind that providing this service costs money (even more so for officers who work 'out of hours') and money is something which local government is somewhat short of at the moment.
Whilst the noise team will aim to attend to every complaint of noise nuisance, please remember that it's quite likely there are only one, or maybe two, teams operating each night for the entire area. If they receive a lot of calls then, obviously, there is a good chance they may not be able to attend every complaint.
However, don't let this put you off and please call whenever you are suffering from noise nuisance. Even if the call is only logged (whether a call from you, or a call from your neighbours who are also being afffected by this one resident) then it is still a record of the ASB and will be taken into account when action is taken.
This could well be quite a lengthy process, but it's better that something is investigated fully, and action taken based on evidence, than on the word of of one person with no evidence.
The likely process is that the council will write to the offending resident based on witness accounts from yourself and your neighbours. If this does not resolve the situation then a Noise Abatement Order, or an ABA (Acceptable Behaviour Agreement) will be given to the resident. This is basically to say that they are breaching the terms of their tenancy and if they continue to do so then they will be liable for legal action which could include anything from seizure of offending equipment to NOSP (Notice of Seeking Possession. Which is basically eviction).
It may seem, at times, like you are fighting a losing battle, and it will be a while before the situation is resolved, but keep at it and hopefully you'll be able to sleep well sooner rather than later
Just a quick note on your thought about using a webcam to record the people in the stairwell. You need to be very careful with this as the stairwell is deemed to be a communal area (as such then it's illegal to smoke there!) and so you could infringe the rights of any people there by making/storing images or footage of individuals utilising the area. If you can set something up which monitors your front door (acceptable) and 'just happens' to cover part of the stairwell then you'll be a bit safer. With this situation then you want to ensure that you don't do anything which could put you in a bad light, or be used against you.
Good luck, and I hope that the situation is resolved as swiftly as possible
