Today I found another crisp packet from 1974 - I found one previously here last September. I've also found fragments of crisp packets which I suspect are similarly old but with no means of verifying their date. I think this is now beyond a coincidence and suspect a container of crisps must have been lost in 1974 and the contents are only now being washed up. (Given the cliff erosion here, it is highly unlikely they have been present on the beach for long).
I will try to look into this further... Incredibly, the print on the packs is scarcely degraded at all, perhaps indicating that the container has only recently shed its contents.