Bike chain help

I hate being the equivalent of someone who buys a computer and then doesn't know what a file is, but can I solicit another snippet of advice?

I have a Sturmey-Acher hub, the chain that fitted which snapped, and on examination has several links broken on one side. The chain which fit the old, broken bike, and the spare chain don't appear to fit.

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An hour of googling doesn't answer the simple question "Which replacement chain should I buy"? Is there a dummies guide which would tell me the basics? I'm don't know which variables are used as opposed to being universal.

Is the hub worn so only one chain fits it? Does the chain only fit because of the broken links? If so, how the hell did it fit in the first place?