It appears there were more incidents than I remembered then, Wrench's AAIB report was the first one I was thinking of but I don't think I ever really noticed the second occurrence mentioned in it. I had completely forgotten about Gipsy's AAIB report for G-SPAU and I don't think there was ever an occurrence report for the other one I was thinking of. In my head, SPAU's incident was different in that on top of the loss of SAS/AFCS, it was dark, snowing and the autopilot had done something that was unexpected by the pilot (heading bug defaulting to North). The two I originally mentioned were fully coupled, day VFR, straight and level when the SAS/AFCS stopped playing, the resultant sudden attitude change seems to have been dramatic enough to cause sufficient confusion to prevent either pilot from realising what had actually happened.
I do realise that the subject accident at Drexell Hill may well have been due to a completely different reason.
Cheers
TeeS