I put the question to the CAA yesterday and had a reply today.
It is aimed at CAT but as an ICAO reference for EU pilot licences it could be used by GA also. As it's free the advice would be to carry a copy.
Seems entirely reasonable to me.
I also asked whether it would apply to a pilot with a UK-issued Part-FCL licence flying, for example, a French registered aircraft to Jersey....which is neither an EU nor EASA Member State.
Or what administrative action would be required for a pilot with a UK-issued Part-FCL flying an IoM registered aircraft to Eire?
My supplementary questions will have to wait until the chap responsible returns from his hols.