How do I tell if a loose chain is because of the chain or the ring wearing? My chain looks as though it's 'stretched' - by the time it's fed around the large front chainring it appears to be 'longer' than when on the other chainrings.

Therefore I'm trying to work out whether the chain/links have stretched, or whether the teeth on the chainrings have worn.

How could I tell which I need to address?
Measure the chain. You can use a chain gauge, or just a metal ruler.

Sheldon describes it in detail, but assuming you have a ruler with inches (and fractions of an inch), you can just measure 12 pins' worth of the chain (under tension). If it it's 1/16th past the inch mark, he recommends changing it.

That linked page also has some information on checking for a worn sprocket: if the chain is new (or you measured and it hasn't stretched), and the sprocket is worn, the chain rollers won't sit evenly between the teeth when under tension.

If you can see light between the sprocket and chain, it's probably time to replace.