A contract can be written or verbal, and so there is no specific level of detail which needs to be included in a contract. A basic contract will encompass core details such as rent, tenants, landlord, and length of tenancy.
In any tenancy agreement there are certain clauses and obligations which are legally implied by tenancy legislation. The law will always take precedent over what is written in a contract, even if there are specific clauses to the contrary. For example, a landlord might specify that the tenants are responsible for keeping the heating system and plumbing in working order; however, this contradicts Section 11 of the Housing Act of 1985, and so the Act would supersede that clause in your contract.
If you have concerns about any clauses in your contracts then it is best to seek advice; students can ask contract clauses to be explained by the Student Advice & Support Centre in the Student’s Union, and you may also want to approach the Citizen’s Advice Bureau for assistance.