Good idea ..... for anyone interested I rang the on call vet Sunday night who said although the stones of damsons are toxic, when they are ingested a compound is broken down into hydrogen cyanide, but the stones must have been crunched and broken for that to occur. Although he'd eaten a significant amount, as he's a big dog he had just swallowed them whole the main concern she had would be a blockage but again as he's a larger breed dog they should just pass through naturally.
We kept a very close eye on him and had a rather worried night with little sleep but he is absolutely fine ... not even had an upset tummy and is still pooping stones