Milesg..
Brake pedal switch..
above the brake pedal arm
under the dash.
best way to see it. lie on your back with your head in the foot well.
looks a bit like an asthma inhalator.
with a small plunger at the bottom end held in until you push the pedal.
picture attached
the whole unit twists to remove and wiring unclips too.
cost about a 10-12 quid from ebay.
on mine it solved the 3 amigos lighting up (T/C ABS HDC)
along with having no brake lights at all.
the switch itself had a internal connector wear out and snap off.
and i was able to do a dodgy repair with a staple till the new switch arrived,
so i keep the old as a spare.
as for resetting the ECU from memory it took about couple of days before the dash lights went away, i think thats something to do with the computer remembering stuff for 20 on.off cycles or might be 50.
but in my case the brake lights started working immediately, which is what i was bothered about.
good luck.
richard.