Yes goes in no problem when engine off
Reverse no problem when engine on
Sounds like clutch dragging, meaning that there is some drive still going to the gearbox when the clutch is disengaged. As Jocko says check free play on the clutc. Also consider bleeding the clutch hydraulics to make sure there is no air in there, I guess that's what Ozzie was checking for when he asked about pumping the pedal, but it's worth doing anyway.
Otherwise, it could be a faulty clutch pressure plate.