The mph .v. kn error is a possibility I shall check that.
I wasn't flying in the aircraft, it was a report from a full-time TP. But in discussion with him and reviewing the data it seems that the errors are primarily happening at high speed (we use GPS as truth data for ASI calibration, using a geometric method). At high speed the aircraft was in a fairly steep dive (1500 fpm RoD and at around 1.4Vh) - it appears possible that at the attitude (steep) that went with that he may have blanked a number of the handhelds satellites with a fair chunk of the back-end of the aeroplane and thrown it's output.
I shall report back further, thanks for your thoughts.
G