If you buy a used car from a car dealer are you covered for the first 28 days if it breaks down?
Regardless of what is covered in your warranty
Answer
It entirely depends on what you bought, what failed, what miles and how much you paid.
if you bought say a 1 year old car at retail price and the head gasket went inside the month then yes you can expect a repair. but if you bought an eight year old £1k car with 70k miles on it and the head gasket went, then that would be attributed to wear and tear. as such would be up to you to repair.
without knowing exactly the problem and the above things like age etc nobody will be able to give you a definite answer.