Thanks intesec
Norton Utilities 15 was bundled with Internet Security 3PC. The two products each have their own Product Key.
The label on the NU CD envelope says "Norton Utilities 15.0 1U 3PC IN" which I take to mean one user, up to 3 PCs and no time limit. Norton Utilities is not registered in my Norton Account, so I can't see any mechanism to manage any time limit.
I'll have a go at live chat support tomorrow.