Bookworm:
I have been doing a bit of checking through the ANO in my spare time (sad, I know).
As far as I can see, you are right that the schedule of radio equipment to be carried unde IFR outside CAS makes no mention of ADF (or any other nav equipment, for that matter).
What might catch you legally is Article 14 (7) which says that any equipment used for navigation, including 'information from selected celestial bodies'(or something like that) must be approved by the CAA. As I understand it a GPS fitted into an aircraft becomes a mod which must be approved. A handheld one doesn't, provided you don't attach it to the yoke.
In addition, Article 15 says equipment carried must be in a serviceable condition and of a type approved in relation to the purpose it is used. If GPS is not approved as an ADF substitute then you are stuck.
Article 16 says that all radio equipment must be approved. Does GPS come under radio equipment?
What I couldn't find were any details on the approval or otherwise of GPS. However, it is clear that you must not descend below MSA except for purpose of landing and on an approach procedure unless you are below 3000' AMSL and clear of cloud and in contact with the ground. As far as I know there are no approved GPS approaches in this country.
So we are not much further legally, but I still stand by my original comments. Sorry for the tedious type of reference to the ANO, but you started it!
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"Take-off is optional, Landing is mandatory"