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It will almost certainly be a small allen key fitting. Do not lose the screw, I speak from bitter experience.
Aroogah-
ok, been away, couldn't trust myself to try and solve this beforehand (thought of leaving a damaged / leaky tap alone for a week? No, me neither.)
So, yes, that little screw, easy to get at when you turn the tap to the on position. Dropped it into the sink while undoing, saved by having the plug in. Then, couldn't get the tap off. Wiggled it, tapped a bit with a rubber mallet, tapped a bit harder. Nothing. Could the spline fixing be gummed up with limescale?
Feel like I'm taking the piss here asking like this, but am now at that 'perhaps better to call a plumber stage' and would feel pretty stupid if he/she came around and executed a little secret jiggle to release it :-(
crossedthread
Ah, good spot. Right, need a little mirror to see it. Cheers @Aroogah, will check back in with the result.