Wife was in the shower this morning when it stopped working. Went down to
the consumer unit to discover that it hadn't tripped it's own individual MCB
but it had tripped the 30mA RCD - reset RCD and all other circuits fed by it
are fine; shower solenoid valve operates so that we now get water flow but
water doesn't heat up at all.

First point - I know that there is a thermal cutout but can't see any
obvious signs that it has operated, nor any obvious means to reset it if it
has - or is it a self-resetting device, ie, does it operate until the
temperature drops down to safe levels and then reset itself? Any way of
testing these?

Second point - Would a thermal cutout problem trip the RCD? I'm no
electrician by any means but my very limited knowledge of RCDs and the way
they sense imbalances is sort of pointing me towards a possible heating
element failure, ie, element casing fails and water gets into
lecktrickery-carrying parts allowing current to leak to earth, thereby
tripping the RCD.

I suppose this is easy enough to test for by connecting a multimeter between
the copper body of the heater can assembly and the heating element
terminals, but what sort of values would indicate a faulty/non-faulty
condition?

Could it be something else entirely?

TIA,

Pete