"I have a GPS III Pilot, which I use purely as a secondary navaid, and wondered if it is considered acceptable to use this as a substitute ADF?"
Of course not, an ADF is a LF/MF direction finder that can only be used in conjunction with an NDB. The latter is a Point Navigation Aid which permits you to know exactly where you are when you are in the overhead.
GPS is an area Navigation Aid which whilst generally being of high accuracy, gives no indication of when it is not accurate. It lacks the basic simplicity of an ADF/NDB combination which provides you with a positive fix when you are directly over the NDB.
All area nav aids must be cross checked with an alternative nav Aid, in the case of GPS, there is no other aid with the same potential accuracy against which it can be checked.