Just happened to be watching this live on ADS-B yesterday, as a mate had sent a photo through of the 4 of them passing over him in Wellingborough....
It was the last of the 4 to land that squawked 7700....one of the pair to spend an hour or so orbiting Gloucester, the previous 3 to land had all also done a run and break to land as well at Fairford, but the last one landed straight in, but it didn't start squawking 7700 until after it ended the orbits of Gloucester and had started its decent.