The fan assisted oven I`ve used for the last 10 years seems to me to behave like a conventional gas or electric oven, ie it`s hotter at top than bottom despite the theory. Cooking instructions for fan ovens usually recommend a temperature setting of 20 deg C less than for conventionals. Roasting meats I usually use 155 C, & cook for an extra 1/2 to 3/4 hr than the mins per pound + 30 min timings given. This method cooks the centre without charring the outside I find (even makes good crackling with the right joint). However, a lot of recipes do recommend loosely covering with foil if the outside is obviously overdoing.