Start with the conditions, not blame.
Mould indicates that a surface has stayed damp enough for growth. The reason may involve water entry, a leak, ventilation, cold surfaces, heating, occupancy or several factors together. A fair investigation records conditions and property defects before deciding on the cause.
Keep a record
Take dated photographs, note the rooms, timing, weather, heating and ventilation, and keep copies of messages and replies.
Deal with urgent risks
Arrange the appropriate help for active leaks or urgent electrical and structural concerns. Avoid disturbing extensive mould without suitable precautions.
Investigate
Check the property fabric and ventilation together. A surface clean may be part of remediation, but it does not remove the moisture source.
Check the guidance for your type of tenancy.
Responsibilities and time limits depend on the type of tenancy, location and circumstances. Use current government and local-council guidance for legal information.