Might be able to take you up on that - hoping for a trip up that way in the next couple of months - will PM you when I manage to arrange.
Thanks for the comments - no diagnostic kit unless some way of reading out codes by holding down buttons etc as in some cars.
Might take the lining off and see if the full 4 unit cable is fitted with a loopback in place of voice - prob not given cost cutting at assembly but worth a (hopeful) look and a chance to nose around.