So, after a few bouts of bloody poop and vomiting and blood and fecal tests our vet has returned a presumed diagnosis of low grade, recurrent pancreatitis. I say presumed as the blood test and the symptoms point towards this being the problem, however the fecal tests, which were taken when Potus was feeling okay as his bouts have not been frequent enough, thankfully, for us to have gotten a sample of his bloody poo, could not 100% confirm the diagnosis.
Nonetheless we are going forward with treating him in line with this diagnosis. We'll be putting an enzyme replacer on his food and he is to go on the Obesity Royal Canin food- despite not being fat- it's purely for the low fat content, though I do feel bad for him having to be put on obesity food when his weight is fine. His treats are now peas, green beans and carrots and he hardly seems to notice the difference!
I'm hoping that this new regime will settle his bouts of bloody poop down. Thanks to all who gave advice and shared their own experiences a few weeks back!