It’s quite simple really.
To have a brand that lasts 50 years and appeals to a wide-range of customers you need to:
a) constantly re-invent (re-generate) yourself
b) but keep true to your core-brand characteristics (keep running)
c) ensure that each re-invention has one iconic element (hair, scarf, northern-accent, bow-tie)
d) but you also keep a consistent iconic element (screwdriver, TARDIS)
And, you need ensure that there is always a mystery… Dr Who?
Hat tip to a bloke who also has a great northern accent.