If the engine continues to increase revs when you accelerate in 5-6th gears but does not pull away in accordance with the extra power, you may have a clutch slipping problem to deal with. Otherwise, I reckon that you may need to clean out the Exhaust Gas Recirculation (EGR) valve as it could be sooted up, which can affect performance as you describe. I think that the time has come for you to get some practical mechanical help to analyse the problem(s) etc. BTW, how many miles do you think that the engine has done and has it been serviced as it should - ie annually @<9k miles with synthetic 5w/40 (diesel) oil being used?

Its starting to not have power occasionally in 5-6th gear when accelerating away & I think it may be the clutch thats on its way out??

Could be a boost leak between the turbo and the engine-although this usually manifests itself in black smoke.

I have a nice simple clutch test,there are risks.

Pull the handbrake on as hard as you can then in 1st let the clutch up and try and pull away,it should stall,if it doesn't and revs up then the clutch is shot,BUT,this will probably kill what little life is left in the clutch.