It needs to be remembered that there are different types of painkillers. Some work to reduce the inflammation that's often the cause of pain, whereas other block pains signals but do nothing about inflammation. So the former type might be most appropriate if there's inflammation present but the latter type might be better otherwise. Your doctors will know more than anyone here (including Sqad) does about the actual cause of your pain, so he'll be best placed to know which
type of pain killer might be best in your situation.
However you're already using one of the strongest painkillers available, Fentanyl, in the form of the Matrifen patch. It's an opioid, which means that it's in the same class of drugs as morphine and therefore extremely strong.
Rather than concentrating on drug treatments, it might be worth talking to your GP about alternative pain management techniques, such as CBT:
http://www.healthtalk.org/peoples-experiences/long-term-conditions/chronic-pain/nhs-pain-management-programmes