Danny - I've been offline for a couple of days, and I didn't realise my PMs were full. As to your problem, there is no central register of tanks and you cannot tell from the number who owns it, even though that number is supposedly unique world-wide. I say "supposedly" purely because there has been a suggestion that the Chinese may have been manufacturing "forged" tanks with numbers duplicated from genuine manufacturers. I hope that suggestion is spurious!
There is a longer-term way of perhaps identifying the true owner of stolen gear. The most widely used discussion forum world-wide for scuba is
www.scubaboard.com, and they have a sub-forum for reporting stolen gear. You could post details of the gear there and see if anyone responds.
Embossed into the thick metal on the shoulder just below the neck are many numbers and letters, giving the specification and usage of the tank (though you need special knowledge to interpret this correctly). The information presented is standard across manufacturers but the manner of presenting it isn't. The key information is the manufacturer's name and the number, both of which can be identified by a process of elimination. There are two or three principal manufacturers. The "number" is usually a letter followed by string of numbers (I can't remember how many and don't have a tank with me).
Sorry if this has missed your deadline.
Peter