Cheers guys.
Just taken the selector mech off, and found the roll pin sheared, and as davey said, the mech on my mk2 box is different.
One of the rear mountings has been smashed and previous owner used some sort of resin to bodge it, so I think the best option is another box. From £30 locally on ebay, it's not worth the hassle of repairing the old one.
Cheers again for the input.