Practically, a tenant should report a slow or blocked drain to the landlord or agent in writing as soon as it appears. Delay makes it harder to argue the cause and can turn a clearance job into a damage claim.
Landlords should keep records of drainage maintenance for the same reason — it demonstrates the system was in working order.
For HMOs and blocks, a scheduled maintenance programme is far cheaper than emergency call-outs and protects the landlord's position if a dispute arises.