Okay, this IS indeed a very low budget film; that is pretty obvious. And, usually those can be screwed up VERY easily. However, I honestly found myself quite enthralled and very much drawn into this SERIOUSlY F'd up little community.
First and foremost, I cannot say enough about the excellent use of sound in this film. I noticed a number of similar comments about this from other reviewers here. EXTREMELY effective... I personally really love it when they get super creative with the sound design in Horror films because when it is done right, it really can add to the mood and atmosphere, around which this movie is solidly constructed.
You notice that in all the positive reviews here, that just about EVERYONE mentions the word 'Creepy'...? Well, that is because that is PRECISELY the word that embodies this film. Another thing that I personally appreciated too, was that this movie could have very easily descended into the mind-numbingly boring and ineffective 'Torture Porn' territory. For my taste, I am really glad that they used a measure of restraint so that we could more fully enjoy the type of mood that they were trying to create. And, in my lowly and wretched opinion, I thought they did a very good job with what and who they had. The setting was definitely there to exploit the violence for it's own sake. But, thankfully the film makers were intelligent enough to see exactly where the strengths of this little low budget film were and emphasized them appropriately. Man, ONLY in the UK can they make films like this (ones like 'CITADEL', for example) where they create this powerful, almost otherworldly Mythology about these types of neighbourhoods that is most effective.
Also too, a person who watches this film MUST realize that at times they are using Pitch Black humour, so dark it is almost opaque. The idea, which I'm not going to go into here because it is sort of a Spoiler, but the main premise is certainly unique, something that I have never thought of before, that's for sure. So, this is one of those movies where they are building a mythology and premise that one needs to be able to accept in order to enjoy it. IF you are a hard core 'Realist' and want ALL of your movies to be FULLY realistic and if you are one to roll your eyes if ANYTHING at all strays from what is 'Believable', then you will definitely not like this film. As another good reviewer here said, you MUST be able to suspend disbelief and really allow yourself to get pulled into the story in order to enjoy it, and I completely agree...
I cannot stress this enough, but IF you are the kind of person who likes these kinds of films, then TOTALLY ignore the meaningless 3.x rating and give it a try.