OK, thanks.
For anyone reading the current (10th) edition of CAP 168 (July 2014), it's paras 3.142 and 3.143 on Page 110, URL as above.
I suspect that technically the apron does conform to those criteria, as there doesn't seem to be any physical reason why aircraft couldn't be turned through 90° on pushback so that they are already aligned with the taxiway before the tug disconnects. There's more clearance (about 50 m) between the stand and the taxiway than there is on many of the T5 stands at LHR.
Does anyone who is familiar with Gatwick know why straight pushbacks from that stand appear to be SOP ?