Well I was there on the day in question and the height reached was 27,061 feet or thereabouts. The CAA had been consulted and the nearby airway was kept clear for the attempt.
G-BONP took off from the Thorpeness Cricket Club pitch [ A regular CFM airstrip] and David used my sailplane oxygen system for the flight. He landed with the canopy covered in ice and the engine stopped. David Cook was so cold he didn't light his pipe for some time afterwards.
The record has been beaten since I think.