The files are from the UK Public Record Office, most likely AIR 20/2022, cant be sure. AIR 20/ 2019, AIR 20/ 2020, and AIR 20/ 2023 are also good files for information.
Thank you for your answer Neil but Kew is far from my home !
Some times ago you post in your site WW 2 aircraft performance, a file on RAF strength in squadrons service at the beginning of november 44 to may 45 for UK and NW Europe I think.