Reich mixer tap 1997 symphony

The hot tap on the Reich mixer tap on our sink has come unscrewed.   I can screw it back on, but it comes off again when you turn the tap off.   There is a hexagonal plastic nut which fits inside inside the tap knob which has also come loose - I don't know if that is part of the problem.   I suspect the solution is simple, but I can't work it out!   Any ideas?

Hi Graham

If identical posts are made in two or more sub forums it always causes confusion, so I'll lock this thread and ask members who may be able to help if they would reply on the other one, here.
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