What can be done about severe arthritic pain. Physio and painkillers have little or no impact?

It depends . I highly recommend a consultation with a pain specialist . A com pleats review of all your treatments and Meds is in order . Local injections into a painful joint may help . Regional pain injection such as those into the spine may help . Radio frequency ablation is a procedure that can help control regional pain . Don't try to control your own pain . Sometime , reducing anxiety will also help. .
Need a good eval. There are a number of treatments for arthritis. A quality history and exam are an essential first step. Then your doctor can determine what is needed. Options include various medications and injections. Sometimes a brace and/or therapy cane be of benefit. Otherwise surgery may be of benefit .
Team approach needed. Have you had recent blood work checking adrenal system and extended thyroid w/u? You may need mental health evaluation. Therapy and perhaps an anti-depressant may help. Also depending, anti-convulsants like Neurontin or Lyrica (pregabalin) or Topamax and others as well as other psychotropic meds can help. How are you sleeping? Sleep-study may be needed. Complementary medicine e.g message etal can help.