Gatwick Piers
I was a frequent visiting spotter at LGW in the late 60s and worked there for a few years (Gatwick Handling and BIA) in the mid 1970s.
The North and South Pier roofs (not the Centre Pier) were open to spectators in the 60s but by the early 70s this was stopped due to a mixture (I believe) of security concerns and difficulty of access as the terminal/admin buildings were extended. The rather basic piers were then gradually replaced with more up to date structures.
It was a great place for spotting when the piers were available with close access possible to most of the aircraft and an excellent view of landing aircraft on 27 (now 26L). Probably hard to imagine now, but the biggest complaint from spotters then was boredom as there could often be significant periods with no movements at all (20 minutes between any activity was common - much longer sometimes) and then it was often just 'common' types like Morton Air Service Daks, Dan-Air Ambassadors, BUA Viscounts, BUIA Heralds, SAM DC-6s, RCAF Bristol 170s and Yukons, etc.
Aaaaahhh! 'You don't know what you've got 'til its gone!"