If this is the case, how would I go about stopping this behaviour? We've throroughly cleaned the areas he goes toilet on.. He did it again this morning - right after he'd been out and gone to the toilet.
What is going on?

Hi Gemma Do you have another dog, if so is it a dog or a bitch .
I would say at eight months George is a bit young for scent marking territory particularly as you say he is a bit shy although he is just reaching that stage where he gets the hormone surge of adolescents. If you think he is scent marking you need to clean the area well by washing with a strong solution of a none ammonia based cleaner that breaks down protein like biological washing powder. When dry rub with a few drops of pure oil of lavender (chemists and health food shops sell this).

I still think it is a lack of house training and would proceed as described above

George is a dog and has been neutured, we're doing well so far.. 2 hours nearly and no little accidents.