Thank you for your reply. The blood was actually quite bright in colour and full of "seed like" material the vet said was probably stomach lining. It did smell incredibly foul though, akin to the smell of anal glands. We can also rule out poisons e.t.c as with two other dogs, two cats, and a three year old we're pretty diligent,
Definitely no dead animals or birds. He wasn't one for eating plants or anything like that either come to think on it. We did check the vomit (it was everything he'd eaten that day) and the stools for anything odd but there was nothing to be seen. The previous day's stools we still in bags too and we even checked those. I know I'm asking the impossible but I do appreciate all of your suggestions.
We know there wasn't much pain in his belly earlier in the evening because - as usual - he wanted it stroked and rubbed. I guess we'll never really know. We're just doing the usual "looking for a way to blame ourselves" guilt trip even though between the first bleeding and getting to the emergency vet took less than 40 minutes. We're just shocked at how fast it's happened when he was doing great prior.
Thank you for your time.