Frits, I had those off when I dropped the subframe over summer, but... I dont remember giving them a super hard look over so will do that now, thanks.
My thoughts...
I havn't had the pressure plate off of the flywheel yet, but it looks
ok in there. It looks to me as though stress on the clutch arm has
caused this somehow though not sure quite how. Thinking back, I've
been thinking the clutch has been high for a while and that I need to
adjust it, but a quick think in my head makes me think this is
opposite to the symptom of that crack being there.
For reference, on this pic

My left hand is at 1oClock, right hand at 10oClock, from crack at 12oClock
12 - Crack
12:30 - Hole that clutch cable outer stops in, activation arm behind
1 - Left Hand, obscuring tiny bolt that holds cover/inspection plate
onto bellhousing, doesnt hold bellhousing onto block
3 - Other tiny bolt as 1oClock
4:30 - Bolt onto block
5:30 - Bolt onto block, then 2 of the 3 starter bolts can be seen
7 - Bell housing to block bolt
7:30 - Crank position sensor bolt1
8:30 - Crank pos bolt2, bell housing to block bolt
9:30 - Bell housing to block bolt
11:30 - Bell housing to block bolt
So... Only 2 of those critical bolts, rather than other things bolts
(4:30 and 5:30) hold the entire thing onto the block, soooo. I think
that 6 till 12 are holding onto the block, and the actual useful bit
has been hinging about the crack point which is what caused my prop to
go too.
So, next question... apart from possible tortoises (which is currently the only thought of theory that makes sense) why did it crack? I reckon I may have hit VTack
