Anyone know what size thread is on the..
 

What size thread is on the clutch slave cylinder hose???
on both sides?

IIRC it changes between SA and FB, as the brake fittings do. Not 100% sure though.

For a series 3 car, I know it's 17mm at crush washer end on the slave, and it's a 10mm flare wrench for the compression fitting on the firewall.

Looks like SA's had a 1.25 thread pitch, and FB's had a 1.0 pitch

SAs are metric coarse, FBs are metric fine. Looking through my tap and die sets, it's most likely an M11X1.0.

Old School Through SA is 10MM x 1.25
FB Forward is 10MM x 1.0