Not sure how but I had trouble finding 6th this morning and there was a slight crunch from the gearbox, anyway when I go back to the car tonight i can't engage Reverse, When I put it where rev. should be there is a slight grinding noise but nothing happens, if I then pull back on the lever really hard it seems to find something but when I start to lift the clutch up It starts to move back but then I can feel the teeth popping out through the gear stick and it dis engages.

I've checked the Reverse cable and linkage all seems to be working o.k so am I right in assuming this is a gearbox out job

 

sound as if it is internal in the box

 

Make sure the reverse interlock cable isn't dying first....

A crunch in top could just be a tired synchro ( like mine ).

dunx

 

Sounds like the synchro teeth on the reverse idler are chewed. This can happen if the reverse check cable plays up and allows selection of reverse instead of 6th. It maybe something else like a broken selector fork for reverse though. Either way it will be a box out job, but parts are unlikely to cost more than £100 (but you may choose to sort synchros out while it's out).