Another update.
So it seems I bought a duff injector despite it being supposedly an OEM item. I swapped this injector to cylinder 1, cleared codes, drove the car for a while and sure enough got an injector fault code on cylinder 1.
Bought another injector (£42 Eurocarparts with a discount code), fitted this afternoon and so far it all seems good. Car feels better as well - though that could also have something to do with the new K-Tec super sports exhaust I fitted a couple of weeks ago, it sounds lovely.