Hi BOAC,
It seems as if the FDs, when 'valid', were reacting to the existing rates which is not the way I expect - ie 'Flight Followers' rather than 'Flight Directors'.
Correct. This is probably the most confusing part about the FD logic. The FDs are withdrawn automatically during unreliable airspeeds and the FMA modes are lost. If two airspeeds erroneously agree, then the FDs will reappear with present HDG and instantaneous VS. In the confusion of the moment, it appears to me that this crew then followed these new FD modes.
See
EASA Airworthiness Directives Publishing Tool and download easa_ad_2011_0199_R1.pdf (21 kb) for a full explanation.