Ok, have a look at this pic:-

If the same thing has happened, and I'm certain it will be the same, the sleeve, item 19, has moved onto item 24......you've engaged 3rd gear. Unfortunately, what's happened is that items 21 have ridden out of their slots in the hub (item 20) and are now preventing the sleeve from returning to the neutral position.
In photographic form, sad I know......but I am fixing Marcus's at the mo and I know he wants to see some pics too, so its rather convenient:-
This is neutral between 3rd and 4th:-

Unfortunately, I have gone the wrong way with this, duh, but it works all the same........this is 4th engaged:-

and in both of these you can see the key seated properly:-

This is what has happened, the key, well all 3 of them, have ridden out of the slot:-

Therefore preventing the sleeve returning to neutral

Hope that helps.........
And.........for your benefit Marcus, thanks to those tools arriving today, I have competely dismantled your mainshaft and am now awaiting the parts from you:-

Oh and we were right, your 4th gear synchro ring is definately f@cked, check out the metal debris that the red lines are pointing too........good job I've got a good spare

:-
