When I was removing the old dimmer, I am pretty certain that the three
wires (red, black and uninsulated) were wired to the three terminals on
the switch (C, L1, L2 - it was a 2-way switch used as 1-way).

Something not right there. Sounds like a muppet fitted it, or it
appeared to be connected like that but in actual fact wasn't. Was it a
brass dimmer, was the uninsulated (which should be sheathed with
green\yellow sleeving, actually connected to an earth terminal on the
plate?

No, the dimmer was a plastic one fitted onto a metal backbox. I am
pretty sure none of the three wires were connected to the terminal on
the backbox.

However, it is not impossible that my memory is wrong.

Alexei