Tim
The stoplight is linked to a number of engine sensors, oil pressure, water
level,anti lock failure, low pressure in air con are a few. You usually get
the stop light + another light i.e.. oil pressure. If the air con has
recently packed up this is likely the cause. Turn off the air con or if it
is one with climate control adjust the cab temperature to the outside
temperature so the air con does not try to come on. This should cure the
light.
After thought the stop light should come on as a test before the engine is
started.
Mike

Check the brake and steering fluid levels.   These are both monitored.

We had a very similar issue with a meridian on a V plate.  Tunred out to
be a bad connection somewhere in the tempererature sensor wiring causing
the stop light to come on and the aircon to not work.  It went away for
a while when I cleaned (i.e. fiddled with) every connector I could find
under the bonnet, but the garage tracked it down in the end.

Chris