Our Triton T80si has started to drip from the outlet (that feeds the shower
head). The drip continues with the electric supply turned off.

There are no obvious internal leaks when I remove the outer casing.

Searching the web suggests a number of things that it MIGHT be, but I can't
seem to find anything which either (1) suggests what it is MOST LIKELY to
be, or (2) a set of diagnostic tests/procedures to go through to determine
where the problem actually lies.

So I'm throwing myself on the collective of uk.d-i-y for any suggestions.