Again thanks for all the responses.
I believe the answer to my original query lies within a statement from The EGPWS product spec (and I must say that Honeywell are being very helpful) which states:-
Terrain is NOT shown if it is more than 2000ft below reference altitude and/or is within 200ft (400ft prior to –218) of the runway elevation nearest the aircraft“
This would seem to indicate that coastal/island airfield would not show a too accurate coastline depiction.
Loads more reading/research to do though