Ok, never mind. Sorted it. It was to do with the little balls and springs... Removed them all, noticed that one of the balls was actually stuck to the end of the bolt with the spring compressed...
Tested the gears without them in (not splitting the case... Again!) And they moved very fluidly. Even reverse. Then used the old bolts, springs and balls from my old gearbox, torqued them up and she still goes though the box beautifully.
So it was those balls. Good to know anyways... Getting excited now to drive my old civic again!