The Air Force List was last published in 2007 and the Retired List in 2006 (IIRC).
Although the Air Force List shows all officers and substantive warrant officers or master aircrew and also lists some senior public servants it is always just that little bit out of date, even when just printed.
The Retired List, however, only lists those commissioned officers who RETIRE; that is they - generally - held a permanent commission at the time of their leaving. An officer on a short service commission, who leaves at the end of that engagement, is not deemed to have retired but merely reached the end of their period of service. There are some variations, as with everything but not too many.
Perhaps an easy way to resolve the problem is to contact the Flying Wing Adj at Valley, explain the issue and just ask him to look at the course photos on the wall between dates A and B and let you know if P/O X's name appears. You can tell him that the info is essential to clinch a major commercial deal and if he wants you to write you can do so. Reassure him that you don't want to know anything else - just was this chap actually there! You'll probably get a blank reply but it's worth a try. They do have a community relations officer who might be a better bet.
Old Duffer