I couldn't be sure what the Morrison oven looked like or how thick the base is. I tried to get an image off the internet but no luck.Or should I cook at a lower temp for longer ?
Flying blind, you have to make sure the bottom stone is up to temperature, really hot. For a Neapolitan, that pizza should be done in a few minutes. That means the base has to heat up well first. Thickness of the bottom stone is an important factor.
For example, the directions I have for a pizza stone in a conventional oven is one hour for preheating. Translated to an outdoor machine, should be less but still a key component.
Worked in a pizzeria for quite a while and the bottom would cool when we got busy in relation to the ambient temperature of the oven because the pizzas sucked the heat from the thick stone. This wasn't a fired oven but a NY style cooking stove.
The thickness of the base is quite important as it relates to being able to retain the heat--thicker the better.
For me, more info is needed about the machine.