Definitely 'That Certain Smile'. Compo, Clegg and Seymour visit Clem Hemmingway in hospital, and buy him a small gift in the hospital shop (totalling no more than 50p)
"A keyring. It is. It's a plastic keyring. Well, fancy."
"Fair makes me wish I'd brought some keys..."
It is, of course, the first time we see Clem 'Smiler' in an episode. The year was 1988.