Reverse selection on R380 Gearbox

Ok, so I thought I'd rebuild my broken defender gear/transfer box. (The spline on the output shaft was stripped). Everything went well ...... except now It's back in the car I have trouble selecting reverse gear. All the other gears are good. I tried bleeding the clutch but that didn't help, as it selects ok with the engine off. (It can be forced in reverse with the engine running and clutch depressed)
I think there is a mechanism which stops the driver going from 5th gear straight to reverse, and I think it's this mechanism which is causing me the grief.
It's a long shot is there anyone else mad enough to rebuild the R380 themselves .... and had this problem.... or even someone who understands how the mechanism works that stops selection of reverse straight from 5th?

As I understand it, this is simply a spring loaded 'plate' that is closed across reverse as you select fifth, and can only open again once the lever has gone back to neutral (3rd/4th alignment), when you can usually hear it spring out of the way again - but never having seen inside the box, unlike your goodself I have no idea how this might have been mis-built on your brave venture.... Maybe spring missing/misaligned?

Hope someone useful comes along soon - can't be much good only going forwards...... A

when you come out of 5th and go across to the left then back across for rev do you hear a click ? its not usual for this to give trouble , baulk ring and worn dog teeth are often a cause of poor rev selection , spring can come off synchro unit which can allow synchro unit to be only moved easily one way and not the other, reverse and 4th baulk ring are different to the other 4 though they all look similar but if it selects okay engine off id suspect poor baulk ring /poor teeth

Thank you both for the information. I'm about to embark on removing the gearbox for the second time, and want to make sure it is the last time!
As I remember the baulk ring on reverse was worn, but reverse selection was never a problem before I rebuilt it. It was rebuilt to replace the main shaft.... (and I put a new baulk ring on 4th, which was giving selection problems).
There is one piece of information I didn't put in my first post, and that is I do get a LOUD click, which sounds like a detent ball being released, it is much louder than it should be, but it doesn't then allow reverse selection after hearing it.
I guess I'll just have to carefully strip and examine the box, and find a good exploded diagram.
Thanks again,

reverse stop can be easily seen with top off box ,its the going into gear engine off that would rule it out for me (unless i mis read that )i do have lr overhaul manual if youd like to borrow it ,is first more difficult too

Mmm. I downloaded some information... R380 Manual gearbox overhaul manual. As you say the reverse stop is at the top, and I never touched it in the rebuild. before removing the lot, I'll take off the gearstick and see what I can. (I'm sure there won't be many good things to watch on TV over Christmas anyway!
Thanks again for the advice.