We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Martin Pentreath
saying something like:

I have a non-condensing combi (Ariston Microgenus) from which there is
a slow but constant drip from the 15mm pipe sticking through the wall
to the outside (which I think is for emergency pressure relief?). The
pressure gauge on the boiler is at 2 bar, which is within its
tolerance range. So why is it dripping and what should I do to stop
it?

If the pressure relief valve is the spring-onna-wotsit-wivva-red-cap
type, try quickly rotating the cap so it reseats. Often a bit of system
**** will clag the seat.
If that doesn't work just replace it, they're cheap enough. Dripping
from these is quite a common fault.