Before I post some updates I'd just like to say thanks to all for their input on this thread. Albrecht - I really wish you were all sitting round the car!!
OK, so I found the arms very tricky to measure. They seemed very similar when I sat one on the other though. In the end I made up (rather shoddy!) jig to see if the pins aligned. Here is the arm from the good side:
And here is the one from the wonky side:
I made some more detailed markings around the spindle pins and proved the two arms are the same. So that's not the problem...
So I removed the strut so I could take a better look at the hub and attempt to measure it.
Based on the assumption that the hub must be parallel to the spindle pin I did my best to measure it using a straight edge held parallel to hub and measuring down to the spindle pin. I did this for each end of the pin. I spun the hub around and measured the distance a few times.
The difference between the 2 distances was consistently 6mm. As a double check, I also took measurements from the rear of the hub with the same results.
To be 100% certain I measured the other hub & spindle pin in situ on the car and it had an even spacing.
So it seems I have a bent hub - full marks to johnymd!