Hi Eamesy79,

It sounds like it has an incorrectly installed belt ,or wrong size ,or return spring problem but difficult to know when I'm not
looking at the parts.

I'd check you have the clutch pulley on the correct side of the V belt and everything looks correctly installed
as usually when the gearbox is difficult to change gears it means the clutch is not fully disengaging (the v belt it not slipping enough)
when the clutch is activated.

Click on this link to check the set up https://outdoorking-forum.com.au/forum/ubbthreads.php/topics/38976/re-rover-ranger-x-c.html

Cheers
Max.