 Cancer patients taking regular medication for their pain often still have episodes of severe pain that break through despite their background pain treatment. Fentanyl is a strong, short-acting painkiller often used to treat this breakthrough pain. Nasalfent contains fentanyl in a patented drug delivery system called PecSys and is given via a simple nasal spray. This study will test the efficacy and safety of Nasalfent compared to Immediate Release Morphine Sulphate in the treatment of breakthrough cancer pain. 