I tried him for a few weeks on each type of food. A week changing from one to another, then 3 weeks on just that food, or however long it took for him to get bored with it, or develop any negative symptoms. The Morrisons My Dog seemed to satisfy him and not cause any problems, and he didn't lose any weight so we stuck with it.
I thought it had started giving him dandruff recently but that was my mistake, it was actually Tescos dry food (all looks the same when you it's in the containers !)
He's back on Morrisons now, and we're gradually introducing various raw meats, including chicken necks, along with some vegetables, eggs, cheese and oats. I hope to have them both on some sort of BARF diet in the next month or so, and if I can get a decent grinder I'll be giving it to the pups as well.