low, med, high is how we useally refer to them.
the click could poss be a relay. you'd have to watch it with the cover off to id that.
their is 2 elements with 4 rongs 2 lives 2 neutrals.
locate each set put a probe on the live one and a probe on the neutral one.
each element should give you between 10 - 18 ohms.
i'm more swung to a fault on the pcb a a faulty element will useally trip straight away when dial set to that mode.
MAKE SURE SHOWERS MAINS IS OFF WHEN TESTING THIS.
hi-spec plumbing & heating (essex)