I've gone back to my PRO notes and found the following, in case it's of interest to anyone:
File MUN 4/2884: Vickers 1.45 inch gun (that's what it was called initially) papers dated July 1916 onwards:
21/7/16: order from Dept Assistant Director of Aircraft Equipment for a "light gun" (50 lbs) of 1.45 inch bore, firing a projectile of 1 lb 1 oz with an MV of 800 fps, fitted with a special anti-recoil attachment for use in an aeroplane. The mounting should be interchangeable with the .45 inch Maxim which Vickers now have in hand, and suitable for the F.E.2B machine.
22/9/16: Military Aeronautics requested 25 Vicker Light Guns plus 3000 rounds including 2000 case shot. It was referred to as a "rocket gun"; quote: "I understand that these 1.45 inch guns are required for firing rockets". Design not yet approved.
28/11/16: trials of gun entirely satisfactory, urgently needed for home defence purposes
29/11/16: DAE request for 100 rocket guns and 250,000 rounds of ammo.
16/12/16: Vickers reports 800 rounds of filled incendiary and 300 rounds of case shot made (denied request to switch to all-incendiary).
5/1/17: Muzzle velocity quoted as 800 fps
8/2/17: "Crayford 1.45 inch" approved by DGMG for aircraft work
12/2/17: "All Vickers Aeroplane and Trench Guns (Vickers Rocket Gun) are to be made capable of firing an AP shell at 1,000 fps".
17/2/17: Fuzes: thought that 1.45 inch shell will take base fuze, so Fuze No.131 not required.