Well, the sound of "a spanner being dropped through the engine
compartment" could have been a rock kicked up from the road into the
engine compartment, and maybe batted around a bit by the fan.

I have no idea how this could have caused the brake pedal to drop to the
floor, unless the rock smacked into a hose or a fitting (vacuum or
hydraulic) and induced a momentary leak. Have you reached in and firmly
flexed everything, looking for a spurt of fluid or a hiss of vacuum?

Bill Jeffrey
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>>>A more detailed check revealed nothing either, no warning lights etc,
>>>fluid level was slightly down, so the only thing i could imagine was that
>>>i'd dropped a disc brake pad or shoe somehow.