Obviously I get it now, but the revelation that Grandstand was accessed via the Boswell's backdoor was mind blowing to me when I saw this as a kid.

That's NOT the "Mastermind" chair, by the way. It's the chair's "stunt double", made for security reasons. After the real chair was kidnapped and held to ransom by students at Coleraine University as a rag week stunt in 1979, the production had a near-duplicate made. Yes, there were TWO Black Chairs. The spare was always kept locked in the scenery van at each outside broadcast in case students tried the same stunt again (which they did). The duplicate chair was never used in a programme, other than for the title sequence. (When the titles were made the real chair was away on location, so the double was used in London). The production team kept their fingers crossed that the Black Chair wasn't kidnapped again while its back-up was being filmed. There are slight differences in the chairs, and I can confirm that Sue Lawley was standing next to the "fake". The real Black Chair was presented to Magnus when the original series ended in 1997. The whereabouts of the duplicate chair not known.

Same day as the news broke during broadcast of the Kegworth Disaster