The landlords serve an invalid section 21 (some pre requisites haven't been met), however, the tenants agree to the move out date (in writing).
The landlord then makes arrangements for travel costs to take the property back on the agreed date, and also puts a deposit down for builders to come and renovate the property (Starting immediately after the agreed move out date).
2 weeks before the agreed move out date the tenants then go back on their word, saying that "a couple of weeks after we put down our holding deposit, the offer fell through". The tenants then find a new place which is due to start 20 days after the agreed move out date, and the tenants refuse to find temporary accommodation before then, which means the landlords must stay in temporary accommodation instead, and the builders cannot renovate the property so the landlords lose their deposit with them.
Who is liable to pay the builder's deposit, and the landlord's temporary accommodation and other costs incurred as a result of these circumstances?