I currently rent a property through your agency at (address).
Last Sunday (date), the heating in the house stopped working. I contacted your office on (date) and advised your receptionist. She told me that it would be repaired within 2 or 3 days. It has now been one week but it has not been repaired. This is an unacceptable situation, especially in winter. With no heating, it is extremely cold. I have visitors arriving soon, and I need the heating repaired urgently.
Please contact me as soon as possible and advise when an engineer will be sent to carry out the work.
I would not include your opinion about having the heating checked every 3 months as it is not relevant to your current complaint. Keep it short and to the point.