Thanks all for your posts. Justine, thanks for the information about your cat - our timings may also be 3 years
Our dog (now 7) was 10/11 months when he came to us, and might
have had an incident before ending up in the Dogs Home. Another possibility is a nasty fall/jump 3 years ago. The vet is re checking old x rays as Finn was xrayed after this event. However, the xray was on a very specific area in the abdomen so the full extent of the damage may not have been detectable, if indeed things were awry then.
Interestingly now I've found it's also called a Diaphragmatic Hernia
I've found a little more about it. Finn appears to have had a Chronic Diaphragmatic Hernia and there is an interesting study here
from 2004. CDH in 34 Dogs and 16 Cats.
We are now 6 days post op and I'm feeling cautiously optimistic. He's eating better than he did before the op which leads me to suggest there have been issues for some time. With the benefit of hindsight there are other possible symptoms which, on their own, were not enough to cause alarm. It's worth noting though that his obstruction would have been unlikely without the restricted intestine and that 3 dogs in the study mentioned were operated because of acute intestinal obstruction.
Big praise again to my other dogs breeder who, in conversation, alerted me to the possibility of an obstruction. Really good breeders, and experienced dog people are worth their weight in gold :)