Death of Chivalry quest - Black Knight armour problem

I need to wear the armour of the Black Knight, so I equip it, but Saradomin still keeps asking me to equip it: what do I do? How do I disable cosmetic overrides? I have destroyed the armour then put it back on, but still it does not work.

There are three different things commonly referred to as "armour of the black knight", there's the Full Black set, there's the Black Knight Captain Armour and there's the Black Knight uniform. For this quest in particular the easiest way is to use the Black Knight Captain Armour, which you have aquired in an earlier quest and lays just next to the cabbage patch Saradomin stands on.

You must equip it, and only it (helm, cuirass, gown, gauntlets and boots).