The lack of cartilage is due to problem number two (or is this a 'which came first, the chicken or the egg?). A proliferation of osteophytes (bone cells/spurs) on the back of the femur head. This has caused the femur head to protrude forward away from the socket and, as the new bone is nobbly, it's worn down the cartilage.
What's next? A hip resurfacing. I have to call to make an appointment but the earliest available time would be easter apparently. I've also been offered a hip block in the mean time - I said I'd think about it. I think it would probably be a good idea though given that treatment could be months away. Scary, giant needles! (have your pelvis realigned no problem, giant needle eeeeeeeeek!). I think I'm just relieved to now know exactly why I'm in so much pain (and what's making that funny crunchy noise....mmmm) and that treatment is on the horizon.