What a puppy contract should actually say
Most contracts you will be handed are one page and protect nobody. Here is what a good one covers, and which clauses are not enforceable.
Almost every breeder hands over a contract and almost nobody reads it until something goes wrong. Read it before the deposit, because that is the only point at which you have any leverage.
What a good contract covers
Who the parties are, and what is being sold. Full names and addresses, the animal's microchip number, date of birth, sex, colour and registration number if there is one. A contract that does not identify the specific animal is not a contract about that animal.
The price, the deposit, and what happens to the deposit. Is it refundable, and in what circumstances? A deposit that is non-refundable even if the breeder cannot supply a puppy is not a deposit, it is a fee.
Health. What has been done: worming dates, vaccinations, vet check, microchip. What is guaranteed, for how long, and what the remedy is. "A 48-hour health guarantee" means you have two days to get to a vet, which is reasonable. A congenital-defect guarantee running to a year or two is better and is common with good breeders.
Return. A good breeder will take the dog back at any point in its life rather than see it go to a rescue. Many contracts say so. It is one of the strongest signals you will get about who you are dealing with.
Endorsements. Kennel Club endorsements restrict breeding from the dog or exporting it. They are legitimate and common on pet-quality puppies. What matters is that they are disclosed before you pay - the KC requires the breeder to tell you in writing - and that the contract says what would have to happen for them to be lifted.
Clauses that are unenforceable or unfair
"The dog must be neutered by six months." A breeder can ask, and many do, but a term requiring a specific medical procedure by a specific date, with a penalty attached, is likely to be an unfair term under the Consumer Rights Act 2015. There is also genuine veterinary disagreement about early neutering in large breeds.
"No refunds under any circumstances." You cannot contract out of your statutory rights. Under the Consumer Rights Act, an animal bought from a business seller must be of satisfactory quality, and a term saying otherwise is void.
Breeding-rights clauses that claim a puppy from a future litter. Common in some circles, frequently unenforceable, and a source of enormous acrimony. Do not sign one.
"Co-ownership" of a pet puppy. Occasionally legitimate in show circles. In a pet sale it usually means the breeder retains control over an animal you are paying for and housing. Walk away.
Your rights if the seller is a business
A licensed breeder is a business, and the Consumer Rights Act 2015 applies: the animal must be as described, fit for purpose and of satisfactory quality. If a serious congenital defect appears within six months, the law presumes it was present at sale unless the seller proves otherwise. In practice you will need a vet's report.
A private seller rehoming their own dog owes you much less: essentially, the animal must be as described. That is one reason the licensed-breeder badge matters.
What to do with the contract
Read it before the deposit. Ask for changes to anything you would not accept. Keep a signed copy, keep the payment record, and keep every message. If you use Panjo Pets Payments the transaction record and the messages are held for you, which is exactly the point.