Rear discs on UK 2.0 Ghia 2000

My nearside calliper is rubbing the disc only on half of the outward face, the inner face is fine. New pads were dealer fitted 6 months ago and still
look like new as I do little mileage. I've removed the calliper and pads and cleaned every thing, the piston moves reasonably freely with the handbrake connection to the calliper.
Any ideas, or is the calliper duff?

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