On Thu, 18 Apr 2019 17:32:32 +0100
Harry Bloomfield wrote:

Ideally, I could do with a better way to be sure at a glance, that
the clutch isn't slipping and the deck is being driven round. One
idea I have is a magnet fixed to the edge of a pulley, rotating past
a coil powering an LED mounted on the tractors dash. Has anyone any
other suggestions?

Bicycle computers use a magnet and a reed switch to measure rotation,
but your mower will probably be going too fast for it to cope. An
Arduino with a Hall sensor could count your spinning magnet revolutions,
and is obviously a lot more configurable but with the overhead of having
to build the system from scratch.