Donkey497: If they overbooked and the flight was full they put on another aircraft as a back-up
I presume you're talking about the GLA/EDI/BFS Shuttle?
I worked on the BOAC check-in desk at LHR in 69-70. There was a period in Summer 1970 when for ten straight days there was not a single empty seat on a US-bound departure. Every flight left several customers behind. No compensation, no care & attention, they were simply told 'come back and try again tomorrow'.