I do know that in the late 70's all the Apes films were played on what was called the Apes marathon. They were shown one after the other, so went on until quite late, until the early hours of the morning. I went myself, but did'nt make the end.
Anyway the poster was the "Go Ape", and the image on yours is the same, only stateing at the bottom it was the marathon run. So maybe the daily mirror was giving away tickets, or you could win tickets, to coincide with the Marathon showings.