Many thanks for all the replies guys!
To clarify, having retired last year at 55, I was only getting the AFPS at the time of my tax code calculation. However I've now started some part-time work, so have registered for self assessment for this tax year...
Looking through my paper work I find that I received an Equiniti Advice of Payment in April showing a tax code of 1060L (as one would have expected), with a pension increase of only 0.8%, rather than the full 1.2% (not sure why).
However, in May, I received another Advice of Payment informing me of a tax code change to 560L, so I presume they've decided I underpaid last year...