Illustrative example
A tenancy agreement states that all repairs to heating and plumbing are the tenant's responsibility. The boiler has failed. The landlord's position is simply that the tenant read the agreement and signed it.
Section 12 makes void any covenant or agreement so far as it purports to exclude or limit the landlord's section 11 obligations, and it does not matter how clearly it was drafted or how willingly it was signed. The only escape is subsection (2), and the fact that decides it is whether a county court has made an order, with the parties' consent, authorising the exclusion - a private agreement, however explicit, does not qualify.
The landlord accepts responsibility for the boiler and arranges the repair; the parties sign a short variation removing the clause, and the tenant remains responsible for the cooker and washing machine as before.