Right guys, finally after a frustrating process I've found the problem - the Kooks CATS are completely blown and have blocked the exhausts and this was causing the excessive boost. I cleaned it all out as much as possible and took it for a spin and the power is back and revving up to 6,500pm.
The rear OEM exhaust still has bits of the CAT stuck in it so the plan now is to replace with new high-flow CATS and a full rear exhaust system.
After some research I've seen lots of stories about the Kooks ceramic CATS breaking up, but it looks like the newer GESi core ones dont exhibit the same problem.
So to match with the existing Kooks headers I'm thinking of getting a set of the new Kooks high-flow CATS and a new Corsa exhaust system.
Problem is I cant tell the difference between the Kooks GREEN cats PN#22603300 and #22603400 CATS - any recommendations? or is there a better alternative to the Kooks CATS?
I was going to match them with the Corsa Valved / Axle-Back 4.5 in Carbon Fiber Quad Tips PN#21789CF and CAT X-Connect Pipes PN#21085