Bernie 
New South Wales, AustraliaModel: Crown CG25E

We have almost the opposite problem to Kombi.

Fork works fine in Forward or Reverse, but put it in Netural and the engine builds up revs like your slowly flooring the accelerator. Go to Forward or Reverse and it's fine untill you go back to N and revs build up again.

Had a mechanic out to look at it, and he looked at fuel supply (but the problems still there), I was wondering if it could be a slipping clutch or similar component.

Any help appreciated as my knowledge doesn't go past V-Twins