14 aircraft were delivered to 10 Sqn of which one was later taken to be the RB211 testbed and never recovered to squadron service, leaving an Aircraft Establishement (AE) of 13. The revised AE post-the 74/75 Defence Review was, in fact, 11 plus 2 In-Use Reserves (IURs) - ie 11 aircraft were funded for crews, fuel, spares etc but all 13 remained available for use at unit level. The IUR scheme was a neat device for those of us concerned with tasking, but I was never certain that Eng Wg was quite so happy with it.