The proof will be in the pudding so to speak, for those due to complete this October, if the work is not completed within 6 months of this date.
Our bet at this stage is the builders will continue to say they can continue to come in on time, during the 6 month window. Until they fail to achieve this, they won't entertain discussions around penalties, even with those with it written in writing!
Once we know the builders have failed to deliver, both on completion date or the clauses for financial recognition of delays, we will be in a stronger position to go in force for interest on our money etc.