NEC ECS: If I accept a subcontrcator's programme that is late do I therefore extend his Completion Date

No of course not. By accepting a programme (under NEC3 OR 4) where planned Completion is three weeks beyond the Completion Date - you are accepting that is where they are at, not whose liability it is. The only way they can get Completion Date moved is by notifying a compensation event that shows the delay in planned Completion was due to a compensation event. If they are unable to do that, then it is their delay and they will be liable for delay damages unless they can recover that time themselves.