I went into the Tesco Express on Spiring road on Saturday and asked if the large OSH is in the 3 for 5 offer, was told it wasn't then I argued the fact that the normal OSH was in the 3 for 5 and it wouldn't be 50% extra if the deal didn't stand and he said "Fair enough, pass me the normal Speckled Hen" he then scanned that 3 times and gave me the large bottles for a fiver. Good lad.
I'm aware of the invitation to treat so if he said no I'd have just bought something else, unfortunately it's down to the mood of whoever is working there at the time.