Well, I don't know really. He was fine late last night and had been fine this morning and early afternoon. Then at 6pm, typically after the vet's shut, he did a pooh that was so runny it physically "ran" down the slope of the yard.
He had another srambled egg and pooped again as normal a few hours later.
It's strange, he's not going any more frequently than normal and doesn't appear to be struggling to hold himself. He still does the "spaniel" sniff and spin to find "the spot" to deposit his poop.
Will monitor him again overnight, but I think I'm going to take him to the vet in the morning for a check up. He is 100% fine and normal in himself...if anything he's even more cheeky than normal!