You need cameras my friend, asap.
http://www.newegg.com/Product/Product.aspx?Item=N82E16881523034
I know you're in the UK and I'm not sure if they ship there, but if they do get this right now. And I mean right fucking now. Wherever those kids keep hanging out, just stick a camera there. You'll have 3 more on top of that as well, that's one for the driveway, one for the front door, and one for the hallway your grow room is in. Or place them wherever you feel would be best, these things are amazing though. You have no idea how awesome it is for me to simply look to my left to see four different perspectives that I could only see if I went to those locations myself.
Back when I grew outdoors, this 4 channel system actually saved me from two attempted break ins to my green house. Both times I saw them on camera and ran out there to chase them off. My room has no view of the backyard, so without these cameras I would have been ripped for sure! I swear though, this will be the best $130 you spent in your life, aside from a dog (which I also recommend you get) having a surveillance system is the next best thing. There's nothing quite like looking at 4-16 cameras all at different views, if that's not a way to ease your paranoia then I don't know what is
To make it even better, you can buy a $10 ethernet cable on Amazon and plug the cable into your router on one end and the surveillance DVR into the other end. What this will do is allow you to record the video feed straight to your hard drive. I delete my videos about every week or so and they record on 15 minute intervals and they never stop recording. It's the most amazing thing ever and it even gets better. You can download an app for your iPhone or Droid and you can view the cameras from ANY location on your phone. If I get paranoid while I'm out shopping, I just pull out my phone and see that everything is alright. Another nice thing about the cameras is the information thing I was getting into earlier. You'll be able to see if they have weapons or not, if they do you can be gone before they even get to the door! Or, if they're unarmed you can ambush them since you know where they're at but they don't know where you're at.
As for getting ripped, here's a little info to put your mind at ease. It is pretty unlikely that a burglary on your grow will end in violence. I mean think about it, the cartels and gangs grow their own product. It's far too much effort and not to mention risk to keep robbing people for your supply. Not only that, with how many people that grow now the odds of them being better than you are pretty slim so there's no reason for any scary people to have any motivation to come after you unless you're selling on their turf or something
The only thing you have to worry about here are opportunistic criminals, i.e. punks and teenagers. Both times I caught people they were kids no older than 16, some even looked 12-13. They thought that I wasn't watching the cameras and that they could just come in at 1:30am because I was asleep or something. In any other case they would have been correct. These kids saw an opportunity and went for it. This is going to be the only problem you have to face. So getting a dog and some cameras up will deter a few of these opportunistic criminals, however you'll still have the kids that think you aren't watching the cameras or something. Don't worry too much, they are kids and kids have a tendency of thinking their immortal. This way if something were to happen you'd at least have evidence that you'd be able to use in a court of law (after you ditched your growing equipment of course
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Oh and one last thing, here's something called an IR illuminator for your cameras. This way at night, it'll add to the vision of your cameras and since it's infrared it's pretty much like having a floodlight that only YOU can see on camera! And they look like cameras to the untrained eye so that's another plus!
Best of luck with you my man.