The Landlord must maintain the property fit for the purpose for which it was let. If the Landlord fails to honour this obligation, you may demand in writing that he attend to the maintenance. The maintenance in question must be a material breach by the Landlord such as no water / electricity, a burst geyser or non-working oven. A material breach does not include missing internal keys, blown light bulbs etc.
If the Landlord fails to remedy a material breach you should cancel the lease and vacate the property or take legal action.
If you withhold rent, you yourself are committing a material breach and the Landlord can take the necessary action to collect the rent – cancellation of the lease, court order eviction or blacklisting on credit bureaux.