Honestly?
It depends on how realistic, practical, sensible, thick or pig ignorant people choose to be!
My advice to people is to close when you agreed to close, not when it might, perhaps, suit you to close. If you signed a contract with a closing date, or a time frame, that is what you have contracted to do. And OP had to apply for retention of an unauthorised development delaying the sale
In terms of compromising, it is the vendors who should compromise, take their money on the agreed date and find alternative accommodation. It's not your purchasers concern that the vendor has no where to move to. The purchasers did not agree to a closing date to factor in the vendors looking for another house.
And if people don't close when they are supposed to? Not a lot can happen for 28 days - during which time a Completion Notice will be served. Now everyone is very unhappy, taking the hump, feeling hard done by and running risk that the deal will collapse, or worse, that litigation will ensue
Move out when you agreed to move out.
mf