Gearbox giving more headaches than wife...

Having localised the problem on my 1994 cabriolet to the gearbox and clutches slipping, the offending oil was drained and suitably replaced. New filter installed and rubber seal to sump. Guess what, yep still slips. Any one have any good ideas, whats this "Lucas" gearbox oil additive? If no clues, any ideas where I can get a replacement box and what price. Mine has done a whopping 62,000 miles, this is what you get when you lay them up and the oild gets contaminated.

Cheers.

Ray.