Hi mate, i saw linkage kits on ebay but if you just want to adjust it theres a procedure,
If the box is an 020 5 speed (on 8v motors) you remove the knob and gaiter, select first and then push the lever left and see how much play before it stops - should be 1.5mm. If not, slacken the lock nut on the collar at the bottom of the shift and rotate the collar untill 1.5mm is achieved then lock the collar again and refit the gaiter and gearknob

Hope this helps. The gearbox code is on the service book front page - if its a different kind then pm me and i'll look up the one for yours