Just managed to get a look at the Tokina.
Found the problem fairly simple fix, though not completely ideal.
Moving contacts for the focal length select was out of alignment causing one of the wipers to short on a thin bit of PCB trace between two contacts. (third visible contact down in top pic)
Slight 'adjustment' shall we say of the wiper mount cured the problem.
Think its a bit of the PCB flex is off square and the contacts slightly off too.
Don't think many people have had this issue though have heard of a few cases of the problem.
Anyway that's made my day! Quote of £144 to repair, glad i didn't!!!!