Genius! So is there also unplanned engineering work, where the navvies
just bundle past the platform staff and fix things before anyone can stop
them, then?

Unplanned engineering work is where something fails (such as the
signal failure at Victoria yesterday) and staff have to go down to fix
the failure. Proper protection for staff on the track has to be put
in place, they don't just "bundle down and fix things before anyone
can stop them".
The defective train was later on, around 13.30ish IIRC. It had to be
pushed out (i.e. pushed by another train).