Thems that know would say that any thief worth his salt will crack any container quite quickly. If you lput any kind of lock on it then the fckers will simply drill a hole in (or 'spear' through plastic). In that case you're left with a) no oil and b) a knackered tank. Even wire mesh enclosures can be defeated quite easily. Best advice is to try to hide the tank.
I know in the states you can get "Paint Mines" - that work just like an anti-personnel mine and shock the burglar and at the same time covering them in luminous/indelible paint I'm not sure if these would be legal in the U.K. that you could use to protect your oil container/property, however here is an article that might suggest it's not the decent thing to do in the U.K.
This is the Intelligent Surveillance & Security Guard Robot, an armed sentry 'bot by the Techwin division of Samsung. It was originally funded by the government of South Korea to guard the North Korean border, but it's now on sale to the general public at $200,000 a pop. That's a price that pretty much limits the market to rich, bored, thrill-seeking eccentrics, the exact demographic you want buying deadly military hardware.
The Guard is equipped with ultra-high definition cameras, infrared lenses, image/voice recognition software ... and a swivel-mounted K-3 machine gun. The robot can recognize and target intruders from over two miles away day or night, and can be programmed to either fire on unauthorized intruders perceived as threats, to require a password and only use deadly force if the incorrect answer is given, or possibly just to murder drifters going for the baited malt liquor you left in the alley behind your loft.
Simple CCTV camera that can cover the thieving gits vehicle reg plate.
Now if its just a case of activation for the recording to start then all you need is a extra cam to clearly show to your local police who stole what when and how much was in the tank.