After eight months of nagging pain and discomfort when sitting down, I finally came across Dr Durtnall's name on this excellent website (see Doctors and specialists in the UK, London).
After a previous steroid injection which resulted in no relief at all and no other diagnosis than the usual one of "inflammation" I was vastly relieved even to just see from the digital x-ray done by Dr Durtnall the actual cause of this discomfort - in my particular case calcification of the saccrococcygeal joint which was preventing the full range of movement that this joint and the surrounding muscles need to function normally.
I have been seeing him regularly for five weeks and much to my sheer relief am 80% improved. He is a true specialist in the coccyx and its important function in the body - an area that seems to me fraught with a degree of misinformation and ignorance.
I can recommend him wholeheartedly - do not be told you just have to "put up" with chronic, life-sapping coccyx pain.
Note from Jon Miles:
I emailed more than a year after this story was posted, to ask for an update, as the long-term outcome of treatment is of great interest. In this case I did not get a reply.