What they mean is its too old to hook up to their diagnostic system.
It can be read via the method here:
www.topbuzz.co.uk/info/fault_codes/fault_codes.htmThe socket is below and to the right of the steering wheel, quite likely behind the plastic trim.
Are you sure its not a 1.2 liter engine? Autodata doesnt list a 1.0 for 1996.
I wouldnt have thought it would be the MAF sensor, as a failure doesnt prevent the car from starting and running albeit with slightly reduced power.
Quite possibly the Engine coolant Temperature sensor, a fairly common issue on vauxhalls of that age