Private Pilot's Licences

Following the enactment of the Air Navigation Act of 1920, the Air Ministry issued their “Notes for the Guidance of Persons wishing to obtain Private Pilots’ Licences” which prescribed the following requirements for issue of a Private Pilots’ Licence:
(a) Obtain a medical
(b) Demonstrate competency
(c) Undergo an examination as to rules and legislation
(d) Meet the minimum flying experience requirements
A holder of an RAeC Certificate issued between 1 February 1920 and 31 October 1922 was exempted from (b) above.
A holder of an RAeC Certificate issued after 31 October 1922 was exempted from (b) and (c) above.
It follows that a holder of an RAeC Aviators’ Certificate was no longer necessarily a holder of a Private Pilots’ Licence (probably the case, but still had to comply with parts (a) and (d) above).
Conversely, a holder of a Private Pilots’ Licence was not necessarily a holder of an RAeC Aviator’s Certificate. The holder could have qualified under (b) and (c) other than through the RAeC.

Therefore, the question is – in the absence of an RAeC Aviators’ Certificate – how to prove whether someone did, or did not, hold a Private Pilots’ Licence?

Does anyone know whether there are any Air Ministry or other lists (perhaps at The National Archives at Kew?) of PPL holders during the 1920’s?