broken boiler at end of tenancy

Tenancy Types and Management

The tenancy agreement ends in 12 days time. I have served S21 on time.
I have just been told by the tenants that the boiler is not working, i had and an engineer look at it and he said the part is no longer available so the boiler will need replacing.
The tenants have not paid the last months rent and obviously have no intention of doing so , or vacating the property without court action.
I cant afford the cost of replacing the boiler unless they pay the overdue rent.
What should i do ?