Slipping clutch is going to be a likely cause be go through everything in sequence rather than just looking at one thing and hoping for the best.

ok, ive just noticed that after all the headaches getting new belts on and putting everything back together that i had re connected the hoses to the vacuum valve the wrong way round,.... its much better now,

seeing around 2000 rpm at 30mph, ... i know thats not perfect but still a big improvement

Yep been there, when I purchased my Vario the previous owner had got the pipes all confused and the pipe off the 4 way that should have been connected to the underside of the air cleaner was T pieced into the vac system. Not great.