I think we are referring to the Grahame-White hangar which was left in a dreadful state when flying stopped at Hendon in the late-1960s. Somebody slapped a Grade something or another on it and it was eventually taken in hand.
An equally interesting building at Hendon was the officers' mess, otherwise known as the Claude Grahame White Cafe. This mock Tudor building is now part of the local college. Perhaps surprisingly, for a place which from the 1960s to station closure in 1987 was the exclusive preserve of those immensely boring equipment officers (later supply) the mess was an absolutely super place with a social life to match anything - anywhere, despite being being a few miles from central London. I experienced several interviews without coffee, usually for taking my MG sports car into the main entrance hall during a guest night or playing Christopher Robin with coffee tables I had bought myself.
The station held an Upside Down Dinner in the mess to commemorate the anniversary of the first inverted flight and a similar dinner held at the RAC club. It seemed strange starting a meal with mess games and ending the evening with pre-dinner drinks.
I still have to smile everytime I drive past on my way to the museum and how many people know that the main display halls are built over a group of hangars which are themselves extremely rare.
The night some local scroats set fire to a unique bi-plane whilst the security guards slept twenty feet away, when they should have been wideawake, is perhaps best forgotten!!
Old Duffer