In the US to fly GPS approaches all you need is IFR GPS...
with an AFMS authorising it.
Same under EASA, actually.
EGNOS is nothing to do with this. EGNOS (WAAS in the USA) applies to
LPV approaches only (GPS lateral guidance with a GPS synthetic glideslope). Currently the UK has just one of these, in Alderney,
AFAIK.
And for private ops, there is
no regulation (UK, EASA or FAA) specifying which navigation source should be
used at any particular phase of a flight.
All that stuff about monitoring an ADF (etc) while flying an NDB approach using a GPS is out of AOC ops manuals (which in any case vary according to what the local CAA has approved). It is sensible to check the ADF (like backing up a GPS enroute with VOR/DME) but it is not a legal requirement for private ops. You merely need to
carry a working ADF.
Compared to a GPS, the ADF will do little more than to confirm you are pointing at say Gatwick and not Manston
Who is this Cath B? Does anybody have a photo?