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Clanad and lankeela, thank you both for your speedy responses.
Apart from the smell, there have been no other symptoms, no scooting (such a funny word, and very apt!), and I haven't smelt the smell on the carpet or the furniture or the rugs, so they can't have emptied indoors.
The smell just seemed much like what others have described as being impacted glands, and it seemed the most likely diagnosis-although I could well be wrong!
I didn't think it would be a problem that would just go away on its own, so I am baffled now the smell has disappeared! Jack's pretty confused too-wonders why I keep sniffing near him!
I will certainly introduce some "roughage" to his diet, thanks for the suggestions. He is on a complete dry food, which I think normally suffices to exert enough pressure on the glands on the way out, and it might well be a one-off. I know some dogs suffer more than others, and this is the first time Jack's been so stinky!
Cheers both, Lisa.