What should be in a tenancy agreement?

The names, the address, the rent and when it is due, the deposit and which scheme protects it, and who is responsible for what.

Beyond that: how to give notice on each side, whether pets or smoking are permitted, arrangements for access and inspections, who pays which bills, and what happens with the garden.

Keep it in plain English. An unfair term in a consumer contract is unenforceable, and a term written to intimidate rather than to inform tends to fail exactly when you need it.