Looking at The GLA Roads (No.3) Designation Order 2008 (https://drive.google.com/file/d/1dmzc0AJXf0tw_JL5aRpLNt5nO-dbq-St/view), entry 14 on page 30 says:
A107 (CLAPTON COMMON), both sides from its junction with Stamford Hill to its junction with Upper Clapton Road
So, how is that bit of Clapton Common not a GLA road? I say it is.
Paragraph 2(5) of Schedule 8 to the Traffic Management Act 2004 says:
(5)References in this Part of this Act to the enforcement authority in relation to such contraventions in Greater London are—(a)as respects parking, or any matter connected with or relating to parking, on or adjacent to a GLA road, to Transport for London;
(b)as respects parking, or any matter connected with or relating to parking, on or adjacent to a road other than a GLA road, to the London local authority in whose area the road is situated.
Ergo Hackney is not the enforcement authority for this road so the PCN is ultra-vires and invalid.
@SMPL I see you've paid the PCN but you have my contact details, next time maybe drop me an email first.