I am a bit surprised by the apparent distress to the locals from these 'booms'. From memory from my time at Brawdy as an instructor on both Hawk and Hunter, all medium/upper air exercises were flown well out to sea either to the west or south west. There was no overland area available south of airway G1 and a transit to the north of it was impractical in both time and fuel terms. So these 'booms' would have been generated out to sea well beyond the earshot of Mr Zammett.