Was doing some more searching today on tyre valves and came across this old thread at
http://www.talkmorgan.com/ubbthreads.php/topics/220271/aero-8-s1-tyre-pressure-monitoring-system which said that Brian Miller on 01494 448158 had a source for the valves a good number of years back in 2014. According to google the number is for Miller Commercial motors who do Morgan Servicing.
Am going to give them a call on the off chance they still have said source but thought it was worth asking if anyone had any knowledge of them?