From memory, it was basically to heat up a cast iron pan to get it very hot and then to sit the pizza on top of that, with the other side of it close to the grill element. He also mentioned how you can use the 'cleaning mode' of some ovens to get up to the required temperature.
A description here http://www.insearchofheston.com/2013/05/how-to-make-hestons-... - see "Step 7: Cooking the Pizza"
From memory, it was basically to heat up a cast iron pan to get it very hot and then to sit the pizza on top of that, with the other side of it close to the grill element. He also mentioned how you can use the 'cleaning mode' of some ovens to get up to the required temperature.
A description here http://www.insearchofheston.com/2013/05/how-to-make-hestons-... - see "Step 7: Cooking the Pizza"