In reply to chloelouell:
Do you have a good independent pet supplies shop nearby? If so, go in and talk to them about what's on offer. I've heard good things about Chappie for sensitive stomachs as a short-term measure, but longer-term it's worth investigating what's out there (and avoiding the cheaper stuff that's filled with, well, filler). Some vets will push Hills Science Plan but the ingredients list makes for alarming reading.
For what it's worth my dog is fed on Natural Instinct raw food (a blend of raw meat, vegetables, and various supplements) and thrives on it. If she has dry food (e.g. if we're camping) it's Canagen large breed, which is one of the grain-free premium dry foods.
This site might be useful for assessing the quality of what's on offer if you find yourself standing in Jollyes or Pets at Home and feeling a bit baffled:
https://www.allaboutdogfood.co.uk/
Editing to add: have you had a chat with the breeder about your puppy being sick? Are you feeding the dog the same food that the breeder was using?
Post edited at 21:57