Right, armed with Keiths words I've done a bit of digging and... basically got nowhere!
This safety notice
http://www.caa.co.uk/docs/33/SafetyNotice201103.pdf states "...Public Transport and Private operators should also use the CDFA technique for NPA operations wherever possible." so would imply we SHOULD use it but may (I would reason such as when doing so might limit our chances of gaining the required visual references) use the dive and drive technique.
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2012:296:0001:0148N:PDF this however only references CDFA for aeroplanes (CAT.OP.MPA.115) so would it be reasonable to assume from that the safety notice has been superseded by the newer regulations? I have no idea so have sent another email to CAA asking them to clear it up with some authoritative guidance... I shall share any response I get.