Chugalug2
Your mention of the word Shinfield ringa a bell and on looking at a map of Reading I see a Shinfield Road which is quite near Wantage Hall and I am pretty sure that on the way out of Reading the camp was on the left hand side of this road.
It was a small camp and I would not have thought that it was the HQ of Flying Training Command but perhaps it was only the living quarters.
Can't say I ever saw any Air Marshalls wandering about but had we realised we were in such august company perhaps our actions at HMSO would have been somewhat different!!!.