Well that explains that! Shaft failure. Snapped on the compressor side, the exhaust wheel is still in place (and actually moves) with its shaft attached.
Pumaspeed assures me that they have near 1000 of the X variant turbo's (and 550+ of the X-47, X-47+ and X-47R options) and they can count they the number of outright failures on less than two hands.
So... I don't know. I really don't know why this failed. I've got 3 oil samples to test (filter, oil pan, intake tube before the engine air inlet on the block - whole intake was covered in oil) and hopefully they'll show me that the engine itself is fine. I'm optimistic.
Pumaspeed will sell me a 'rebuild' which can consist of whatever parts are needed which is clearly a few. So, for £424 (shipped) I can get a replacement turbo - most likely the direction I'm headed.