EI-CLI departed Dublin Airport, on a scheduled flight to Edinburgh and was cleared to the Dublin VOR. As the aircraft was flying over the VOR the No. 3 CCM in the aft galley sensed a strong smell of burning. She checked the oven where she had been heating the bread rolls and noticed flames in the right hand compartment of the double compartment oven. The Captain levelled the aircraft at 4,000 ft, declared a PAN and requested an immediate return to the airfield.