As above, if you paid Sydenham's then it's up to them to sort. Don't be fobbed off by them blaming Hobbyweld, it conveniently gets you off their backs. Apart from it being their responsibility, even if you paid Hobbyweld they aren't acting as their agents for nothing. It could also be down to them, if a member of staff has being helping themself, or just bad stock management.
You could contact your local trading standards, I'd think they'd take an interest over potentially dodgy pressurised gas cylinders! If they flog very little it could be the gas has leaked away over a long time. I'd be loathed to return the cylinder if they are being awkward, chances are they'll claim there was no fault & you'll have no option but to accept their judgment. I can't see any justification in asking you to pay for a replacement, isn't their business model that you buy a cylinder & exchange it for a full one? If you pay twice then you should be in possession of 2 cylinders.
If you're worried about falling out with the only local supplier, you could always send someone else to buy your gas from them.