Mr Mustard did not know that Harrow let you use a blue badge which is up to 14 days out of date. This is probably because it takes the pressure off them to renew in a hurry. In the instant case the blue badge was actually 19 days out of date.
This dispensation my exist in other boroughs so if you are waiting for your new badge you might as well put the old one out (even though technically it has no effect). The blue badge holder must be in the car when parked or is being collected soon after parking.