Hugh wrote:
Everything is cross-bonded correctly, but while touching the combi-boiler,
and the timer, at the same time, got a small shock. The boiler is an old
Worcester, the timer a separate wall mounted Honeywell, I had the front of
the boiler off - NOT touching any electrical componants.
Interested to know if it's the boiler/ timer, or the electrical circuit, or
could be either.
I do full gas/electrical installations which are certificated by qualified
persons, so, please, no 'if you don't know what your doing, get an
electrician in' type replies - they are not helpful.

Measure voltage between the two items.
Also, it's winter, and may be low humidity in some places, leading to
static shocks.