If you get a tank off ebay, make sure you have sweet talked some pub owner to have them filled for you first. Or look at a fire extinguisher place.
If the tank is relatively new it should be ok, make sure it still has time left on its certification date.
Personally, I would just rent a tank of argon/CO2 mix. Will get best welds. For a few months would cost about the same or a bit more than buying a small bottle of CO2.
CO2 will work, but requires a good welder. Your Clarke should do ok on it. I have done some nice CO2 welds with my 130EN. But Ar/CO2 gas made real nice welds especially on thin metal.
Air Liquide will rent by the month, check out this thread which has a link to their site, you can find a local agent they have them all over. If you're near Kent (Margate) Adams Gas seem to do the best deals. If you search Adams gas on the forum there is loads of info and steve@adamsgas is a member.