As above. Even though the car has had an alignment the wheel is not central in the arch. That's kind of miss leading, what I mean is both rear wheels aren't equally distant from the fore and aft of the arch. There's about a 5mm discrepancy with the passenger side sitting slightly further back, visually at least. Ideas?

Sounds like you've got it now but surely it has to be the caster! Having the alignment done is fairly pointless without it being done I would imagine, will adjusting it not change all the other measurements? Why wasn't it checked out of interest?

 

yes if you adjust the rear caster you will then need to tweek the rear toe