I have now received a reply to my query to the CAA about my personal licence/PMD combination:
"Thank you for your email below.
Your email has been passed to the GA Unit to respond to. In terms of the "Existing PMDs remain valid for the type of licence(s) and aircraft weight limits specified at the time of a declaration.", this relates to any declaration that was correctly made before the changes were implemented and then subsequently changed back.
'Does this mean that a UK issued LAPL(A) holder is able to continue to fly G-registered non Part 21 aircraft in UK airspace on a online PMD signed made in 2020 (or earlier) till it's original expiry date?
In my case, the licence type ticked is EU Part-FCL LAPL to fly non-EASA aircraft, no greater than 2000kg MTOM. I only wish to continue (when Covid-19 restrictions are relaxed!) to fly non Part 21 aircraft (LAA permit aircraft in my case) atm and have no desire to fly Part 21 aircraft in the UK or any aircraft outside the UK.'
Unfortunately, holders of UK Part-FCL licences both LAPL(A/H) and PPL(A/H) were not previously and are currently not permitted to make a declaration to the 'at or less than 2000kg' criteria. All of the exemptions that were published allowing UK Part-FCL licence holders to declare only referred to the disqualifying medical conditions as per Art 163 of the ANO, they did not refer to the 'at or less than 2000kg' criteria.
The amendment at the beginning of the year which brought the PMD into the UK Aircrew Regulation, also did not refer to the 'at or less than 2000kg' criteria. This means yes the holder of a UK Part-FCL LAPL or PPL can make a declaration but this must be, at the moment, to the 'at or less than 5700kg' criteria. CAA is working with the DfT to establish the use of the 'at or less than 2000kg' criteria for all UK
licence holders."
So not good news for me (and I suspect some others) in the short term. However, the GA unit later added that "We hope to have the review complete by the time flight training returns."