Have finally had the problem solved after taking the car back to the dealer again and insisting a mechanic came out with me to whiteness the problem, car was in for a couple of days and turns out it was a faulty egr (exhaust gas recirculation) valve. The car is now running sweet as a nut again, though glad I have an extended warranty as the part alone is getting on for £500.
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I thought it sounded like the EGR valve at fault
An EGR blanking plate costs £5 or less...+ no more EGR issues / a solid plate "fixes" a faulty EGR valve by disabling it - spend £500 and fix it temporarily, (untill the next time it fails) or £5 and fix it forever.... thats a hard one.... let me think about that........