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Tried changing the calipers from side to side, no difference. Piston not seized. Only option left was the collapsed brake hose as suggested by markyp2000
Bought one from WilcoMotoSave (£15 as oppose to the £36 my normal motor factors was asking). Fitted it along with the new Ferodo pads, and hey presto, nice straight braking and MOT pass!
Thanks a lot chaps, and especially markyp2000. For anyone who is interested or reading this in 3 years time after some help, the brake hose removed looked absolutely fine. A thorough check with a helper applying the brakes still didn't show any issues. It looks original, matched all the others so a difficult one to find.