Everything But Reverse

Brother in law got home yesterday from MOTing his 300tdi Disco (it passed by the way) without any problems. Stopped at his house then came up to mine to work on his Defender rebuild project.

Problem is when he got to mine he couldn't get reverse and it was difficult to get out of 3rd.

Stripped the top off the gearbox thinking the plastic ring had broken, it's all fine so went a little deeper still can't find anything wrong but still won't go in to reverse.

He's going to change the oil today but is there anything else that we could look at with out taking the box out of the car?

in what way ,gear stick wont go over and back or does but jams in going into gear as if foot not on clutch

The gear stick is fine and moves around freely, when we had the top off we tried with a screw driver and pry bar to get it to go into reverse but still no joy, it will go in to every other gear using the screw driver and pry bar and the same with the gear stick.

on left hand side with top off looking into gearbox rear housing there is reverse interlock ,flap type peice of steel which works on selector quadrant its supposed to move up down so that you cant go directly from 5th to reverse, they can jam ,dog teeth are prone too wear but normally gives poor change over a period of time

I know the interlock you're talking about it's in place and ok, just can't understand why it won't go in to gear.

If new oil doesn't work it looks like it have to be another box.

new oil wont fix it may improve gear change , synchro springs can break up jamming syncro go one way but not other unforuneately revs under housing and only 5th is visible