Ok if the fan is running and the oven does get a bit hot then its more then likely the thermostat at fault here. As the thermostat tells the heater to get hot so if its getting hot then the heater is working but if not hot enough its the thermostat knocking off to early.
For you local AEG engineer call this number 08445 610647.
They are the agents for you area for AEG so give them the details and they will come out and get it sorted for you.
If you need anymore info just get back to me.