Thanks for the great advice so far.
As I want to use the medical declaration route, I wonder whether the only realistic option is to convert to an NPPL(A) Microlight for which I would get some credit for my old PPL(A) (I think). If I try and do anything with my old PPL I think I'm right that I would need to revalidate my Class 2 medical and I don't really want to jump through those hoops again.
Following on, if I do the above, does anyone know whether the NPPL would then survive all of the EU transition arrangements such that no other licence conversions would be required (assuming MLs only within the UK)? I think this is the case as MLs are Annex II.
Thanks again.