Don't remember WD canteen actually closing for cleaning, as far as I recall they just cleaned round us if we were in there, usually max of about 6 at a time. I was there Mar '69 to Sep '71.
I too used the QB restaurant courtesy of my wife at the time. She was secretary to the BEA Training Manager Tridents/737s (c1978) and worked on the same floor a matter of 50 yards away, and would 'borrow' a colleagues pass to get me in.