Sorry guys, I can't see this being a Gearbox issue as stepping on the clutch will disconnect the engine from the gearbox and if it was the gearbox, you'd be able to turn the engine back and forth a few degrees due to backlash through the drop gears.
Likewise, I can't see it being the C Clip and this is nested within the flywheel and can't dislodge. If the Primary Gear or thrusts had disintegrated, again, I doubt that would lock it up.
Having said those items, I'm at a bit of a loss here as to what to suggest it could be, but I do think it'll be an engine out and at the very least a part strip. Perhaps before doing this, whip the Flywheel & Clutch out just in case a bolt has walked out and jammed it.
Engine is out, but I've misplaced by flywheel puller, so i'm waiting to source another one.
The engine can be moved back and forth a couple of degrees on the crank pulley.
Edited by liirge, 25 May 2018 - 12:11 PM.