Thanks SSBob - it looks like page 38-19 is the one:-
SECTION 8 -
RANK ON RETIREMENT, RESIGNATION, ETC..
Retention of Rank by Officers on leaving the Active List. Sponsor: RAF Employment Policy
(1) An officer of the regular air force placed on the retired list or on the reserve will be shown in his substantive rank. An officer of the rank of flight lieutenant or above leaving the reserve, or having no reserve liability, may be permitted to use his rank as a courtesy title subject to clauses (7) and (8) and to completion of 3 years service on the Active List.
(2) Subject to clauses (7) and (8), with effect from 1 January 1956, an officer of the regular air force, other than an officer serving on a national service commission, may be permitted to retain as a courtesy title in civil life a rank higher than his substantive rank on leaving the Active List, provided that he had held
such higher acting rank for an aggregate period of at least two years during his service, or continuously for one year immediately before leaving the Active List. For this purpose, service in a higher acting rank, may if it is advantageous to do so, be counted as service in a lower acting rank.
(3) Officers who reverted to the rank of flight lieutenant on entering the former Supplementary List may be permitted to retain the substantive rank they held immediately before transfer or appointment to that List.
(4) As stated in clause (2), these regulations are effective from 1 January 1956, but officers who served during the Second World War, 1939-1945 and for whom the regulations in force before 1 January 1956, are more favorable, will continue to be treated under the old regulations (see clauses (5) and (6)).
(5) Second World War, 1939-1945. The regulations applicable to officers who served during the Second World War, 1939-1945, are as follows:
(a) Officers, on leaving the Active List, may be allowed to retain their substantive or war substantive rank, or, if more favourable, either:
(i) Their highest temporary or paid acting rank, provided that they have held such rank for an aggregate period of at least six months (for this purpose any periods of tenure of such rank will be reckoned cumulatively, and service in a higher temporary or paid acting rank may, if it is advantageous to do so, be counted as service in a lower rank, eg a flight lieutenant who held the temporary rank of squadron leader for four months and the paid acting rank of wing commander for discontinuous periods aggregating two months, may be allowed to retain the rank of squadron leader)
(ii) Any temporary or paid acting rank held immediately before the beginning of absence from duty on account of disabilities attributable to air force service as a result of which they cease to be employed and relinquish their commissions or are placed on the retired list, etc.
(b) Any period of temporary or paid acting rank held by an officer whilst seconded or attached to another arm of the Service may count as under (a).
(c) An officer who, on being taken into employment as an officer during the war relinquished rank previously held, may, on leaving the Active List, resume that rank or may be allowed to retain any higher rank for which he is qualified under this clause.
(d) To be eligible for the retention of rank, officers must have been in receipt of RAF, as distinct from civil, emoluments during their service on the active list.
(e) The provisions of this clause have retrospective effect to 3 September 1939, but service in a paid acting or temporary rank after 10 February 1954, will not count towards the qualifying period defined in (a)(i).
(6) In all cases permission to retain rank will be contingent upon an officer's service having been satisfactory throughout, and the Defence Council reserve the right to withhold or withdraw this privilege at their discretion at any time. Officers who resign their commissions will be dealt with in accordance with
para 3005.
(7) The counting of acting rank for retired pay purposes is governed by separate regulations (see para 2933) and the grant, under the provisions of clauses 2, 3, 4, 5 and 6 of permission to retain higher acting rank on leaving the active list will not effect an officer's rate of retired pay, nor will it entitle him to recall in
the higher rank (see para 3003). It will permit him to use the courtesy title in civil life and to wear the uniform appropriate to the higher rank on occasions of ceremony, in accordance with AP 1358 -Dress Regulations for Officers.3003.