Thanks for the good wishes, one and all.
It's still mysteriously painful - sort of. It's not as clear-cut and cutting as arthritic pain, but seems to run in a line at the moment between my right armpit and the crook in my elbow, and a bit down my right side. It doesn't feel muscular. I've had a poke around to see if I can feel any lymph nodes up, but there is no sign of any.
Oh well, a couple of paracetamol and to try and sleep a bit better than I did last night.