Tommy Sopwith
I think the FI cutting from 1973 about the London Plane Show clears up exactly when the photo of G-BASE was taken.
I thought the picture didn't look much like the City Helistop which was on the barge "William" moored next to warehouses on the south bank, near Cannon Street rail bridge. The location looks too grand for this.
The shot seems to have been taken from the terrace of the Festival Hall and shows the Thames lighter "Mercator" used as a temporary landing pad in the backgound. Note also the elaborate street light.
Shame such an event could never happen again.