Rory O'Conor wrote:
Can anyone advise on any tests air or ground-based, with the instrument
installed / removed than I can carry out to determine whether this
instrument is functionning correctly or not?

I suspect any avionics repair shop will be able to reliably test it.
Personally, I just go thermalling in VMC and look what it shows after
half an hour or so. Be sure to thermal to both sides.

Stefan