Our kitchen sink has a tap head with a switch to change from flow to spray settings. The switch has developed a bit of play in it recently.
I thought I'd open up the tap head to see if it's easy enough to tighten something from the inside.
But I can't get it open. I've removed what appears to be a grub screw above the switch but nothing comes loose or can be twisted.
I've tried applying a little pressure using the screw hole by using a small Allen key for leverage and still nothing wants to move.
How do I get this tap head open?

