E60: 530d Engine-Death Post-Mortem:

Had the rough-running / misfire from the vid in this thread [], so I've quickly whipped the cyl-head off [lol!] and this is what I found...No. 4 swirl-flap has broken off and is missing from inlet-manifold. Some technician could have removed or broken this with the mani. off in the past, but this is unlikely so it looks to have been chewed by the engine. With all that intake-pressure it's possible the skinny flap has been totally disintegrated by the valves / piston and blown through the exhaust in tiny pieces...
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...because there is no sign of it in the head, or inside cyl. 4 / the exhaust manifold. The turbo is spinning great and there is no play in the spindle. You would imagine the flap getting stud behind the inlet-valves, right down the inlet-port, but there is just no sign of it. I got the inlet valves down using a home-made valve-spring compressor, built from a piece of angle-iron and an M10 nut/bolt and sent pipe cleaners through. There is no obstruction in the head at all. Ima
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Slight marking on piston 4 suggest the flap has been mashed through the cylinder, or that the inlet-valves have gingerly contacted the piston while that was going on.
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In the block, cylinders 1-3 appear to be working perfectly. All cylinders are in the right position and the engine was running [badly] so there is no seizing. Cylinder 4 appears to be firing correctly, despite the marking to piston-top. Cylinder 5 appears to be firing ok, but there was a small pool of neat diesel in it. This may have just spilled over from cyl. 6 when the head was removed, as 5 appears to be firing. 

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Cylinder 6 was full of neat diesel to the brim. There was no sign of 'hydro-locking' or the neat fuel affecting the crank, so it seems to have been gushing straight into the turbo/exhaust. Exhaust-port 6 on the head is wet / oily with diesel, whereas the other 5 are bone dry and sooty, so it looks like only cyl. 6 was in fact not firing.

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The top of piston 6 is quite badly corroded, as is the top of the combustion chamber in the head around the valves. I wonder if excessive fuel from a leaking injector could cause this?

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Injector 6 appears fine [as is always the case].

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There is no sign of damage to the head, cams, block, manifold and turbo, so I have to give the head a clean bill of health and will be putting it back on. New gasket is £80-£120 on its own!

No loss of compression on cyl. 6 or damage to head-gasket. 

My best guess is that as exh. port 6 on the head is wet with fuel, no. 6 injector has flopped it in a big way and been flooding the back cylinder with fuel and not firing, which could still explain the sudden nature of the fault.

If this not the case, then I can only imagine the dropping swirl-flap has caused untold damage to the engine, knocked the timing out somehow affecting only cyl. 6 completely, but 4 and 5 to fire improperly too.

Yeah, so looking like a truly broken injector then! Guess I will be replacing all 6 when I refit, about £40 each, along with some swirl-flap blanking-plates for £7.50. I also broke the thermostat-housing as I removed the feed-hose, so that's another £40 down the drain, oh and more oil and coolant of course. Think I'll re-use the head-bolts!