Hamstring strain, actually back-related.
So - after MRI scans in January 2012, free-standing X-rays and private consultation with a back surgeon in March 2012:
1) I've lost 60% disk height in four discs
2) I have a hyper-mobile disk at the top of these, over-compensating for the lack of mobility below it
3) The vertebra at the bottom of my spine is shattering and merging into my sacrum (pelvis)
4) When I lean back, the vertebrae slide over each other.
Basically, my back is screwed. It has always been hyper-mobile and I was running away on it for years before anyone had ever coined the term "core control" - as it was, I overtrained my abs and back and created an imbalance that has pulled everything out of whack. I popped a disc at L1/L2 in 2003 and this was never properly fixed. These things are now not fixable and are only ever going to get worse. The private back surgeon could fix the disk bulges impinging on my sciatic nerves for the grand sum of £10-14000, but the underlying problem will not improve. You can't reinflate discs. So - game over for the fell running, really, perhaps for all running. We'll just have to see.