Chances are it's failure of the front clutch. Either repair just that at your own risk or overhaul the box, either way the box will have to come out to do it. The BW65 had modifications to the front clutch to soften the engagement of drive, but that extra complication makes that clutch more prone to failure than the BW35, but overall the BW65 is a far more reliable box than the BW35. Remove overhaul and refit will probably cost about £1200, repairing just that fault yourself if you have the tools to do it could cost less than a tenner.
PS. I saw that car and thought about having a punt on it, but space constraints, added to the thought of reversing it all the way home put me off.