It's definitely not the relay. I measured voltage across the contactor coil terminals so the relay was doing it's job.

I had a spare contactor so I've been out today and replaced it then taken the broken one apart.

The coil has a thin bit of springy metal sticking out either side of it. The top of the housing has 2 screw terminals and when pushed into place, the springy metal touches the bottom of the terminals.
It looks like 1 of them wasn't quite springy enough and wasn't making proper contact.

I'd guess that more expensive contactors would have better build quality but I wonder if they would still use the same sort of springy metal technique.
I'm relived the coil hasn't completely burned out anyway. It's looking like it may just be a 1 off problem but I'll keep testing them and see if I have any more failures.