Let's say that I pre heat the oven to, say, 240c before putting in the dough.
I then open the oven and lose about 20 - 30 degrees so the dough's going in at a far lower temperature than recommended.
Would it be advisable to pre heat to 260c to compensate for the heat loss?
I think you're right. I always preheat my oven to the highest temperature(about 270C).