Hi Guys, how of you travel into London via the GN lines to Moorgate
and transfer to the Underground? How do you validate your Oyster
Cards? Each time I pass the 'Validating Machine' at the top of the
escalators that link Moorgate GN to Moorgate Northern Line, the
validator displays closed!!!!!

Sound like you'll need to leave the station using your GN ticket and
re-enter using Oyster. (Or get the unstarted journey fixed at the
ticket office of your destination station, or (if you Oyster is
registered) get the Oyster helpline to fix it.)