incidentally my grandfather laughed in his face and said he intended to forget everything about the base as soon as he got back to his home and fiancé.
so far as I can tell he was pretty true to his word. The only anecdote I've got about the lab from him, is that they once put a load of flies in a vacuum chamber just to see what would happen
It would depend on what kinds of instruments were being tested.
You have to remember that in 1954, ANY information on ANYTHING getting across to the Soviets could have major consequences for the West. The collaboration/export of technologies ended immediately following WW2, and both sides went to ground on practically everything.
The UK tested its first nuclear bomb in 1952, so he could have been involved in the testing/development of instruments for testing/measuring nuclear devices. Similarly, the late 1940s and early 1950s also saw the development of second-generation radar systems, something the British obviously would want to keep away from the Soviets.
I'm sure there were many other areas of instrumentation that the RAF would have been developing at the time that could have been in the testing facility which the British wanted to keep hidden from the Soviets.
Its a standard thing for any post covered by the Official Secrets Act(s), ie. any job that requires national security vetting, to be reminded on leaving that you are still bound by the terms of the Act.
Sometimes people actually sign a piece of paper "Signing the Official Secrets Act" at the beginning and end of their employment in that post. Its actually just to serve as a reminder as the Act is law regardless of whether or not you sign it or receive a verbal reminder.
There is still a requirement in the current Act for any disclosure to actually be damaging, so discussing the engineering tolerances of a new altimeter with old Bob down the pub would only be an actual offence if it turned out old Bob worked for Mowscow.