For some reason it seems that Silverstone has a shorter history than people of my era would have thought.
The official Silverstone website tells us:
This year we celebrate the 50th anniversary of the Grand Prix at Silverstone and ask you to share your favourite moments with us
Yes there was a British Grand Prix in 1964 but it was at Brands - and anyway the first time a race at Silverstone was called the Grand Prix was more like 66 years ago.
Having looked at the sub-set of the website, I see that what they are actually celebrating is not the 50th anniversary but the 50th Grand Prix at Silverstone