PJK wrote on 24/06/2018 :
The bit I don't understand is why nothing tripped. The live and neutral
wires are both wired into the contactor and the earth from the lathe and from
the feed from the plug both connect to an earth terminal on the switch casing
and were done up tight. The socket in the garage is fed from a dedicated 16
amp breaker in a modern MK consumer unit with an RCD. Should that not have
protected me?

The RCD will only trip, if sufficient current passes through you, to
earth. Possibly there was not a sufficiently low resistance path
through you to ground to trip it. Another possibility is that the RCD
is faulty or seized up. All are fitted with a test button, does it trip
if that button is pressed?