Landlord's Responsibility for Drainage

Maintenance

URGENT

Dear Advice Team and Forum.

The Situation:

I have let a property using the standard NRLA AST agreement (2020 Version 1.1m)

The tenancy agreement for this small 3 bed property stipulated 5 residents in total, two incontinent due to (young) age or disability.

The tenants invited others to join them (without my permission as landlord) and - with the birth of a baby to one of them - this small property has eventually been occupied by no less than 11 residents, five incontinent due to (young) age or disability.

They provided themselves with a chemical toilet to augment the bathroom toilet. The contents of the chemical toilet (when completely full) were simply decanted into the bathroom toilet as a matter of routine.

Wet wipes and kitchen roll have been routinely used as sanitary products, which most hotels etc rightfully plead with residents not to flush.

The resulting misuse/ overuse of the bathroom toilet has - predictably - resulted in regular blockage of both the toilet itself and the external drainage leading to the adjacent public sewer. The interval between blockages is now down to 3 weeks or so. This situation has obviously involved this landlord in multiple expenses for the unblocking of same, which expenses are now almost totalling the value of the (maximum) 5 weeks deposit taken. The misuse has been recorded in the job reports filed by the attending drainage engineers.

The residents (both authorised and non-authorised) have been made fully aware of the above and have been instructed accordingly.

My Query

Clauses 1.18, 1.22 and 1.38 of the AST require the tenants to take all reasonable precautions to avoid drain blockages etc

Clause 2.6 requires me as landlord to keep drains and external pipes in repair and proper working order.

At what common-sense point - if any - can 1) I decline as landlord to meet further expenses for misuse/ overuse of the drainage system and 2) simply arrange a contractor on their behalf if (repeat if) they are willing to pay for the contractor themselves.

Can I demand an interim replenishment of the deposit or would this be in contravention of the maximum 5 week rent for deposit rule?

Please advise.