Poor baby
I would give the vets a ring to be on the safe side, when I rang them the first time it happened to Riley (out of hours) they said as he was breathing OK, not distressed etc to wait til the morning and see how he was. I ended up taking him to the vets the next morning as it was still quite big, that's when the vet told me about the half piriton but that's for an adult cat of 6kg so I wouldn't give anything to little Archie without checking the dosage of the vets first.
I know of a chemist that's open til midnight if you need to get some Piriton