bad luck for your mate there hidepenny
Take the hint though, keep your bike "out of sight" Most of these thefts are opportunity ones it looks, pricks on the prowl two up or in a ute, looking for a bike to lift. If you keep yours behind a bush or tucked up an ally way you reduce the chances of it going missing.
As for chains, nearly any chain can be snipped in a second or two with a big set of bolt cutters.
Nearly every chain that is

What you want is a length of "crane chain" the kind they use on mobile cranes and the kind the truckies use to tie down dozers and the like. It's as expensive as all shit but if you scope a few crane mobs they often have some old stuff that is not good for lifting loads anymore but is perfect for your bike. The only way you can cut it on the fly is with a gas-axe, it laughs at bolt cutters and hacksaws. Not many people have access to oxy cutting gear and wont be prowling the streets it either. You will need a hi quality security lock to compliment it too though, don't know what they would cost, but quite a bit I'd imagine. Go see a locksmith and tell him what your up to and you might get the info there.