I think I found the main cause of the idle problem today actually. Was out playing around with tools and acting like I know what I was doing!!
Anyway the gasket between the AAC valve or whatever and the manifold (where there's the two bolts from the valve into the manifold, there's a gasket there)... the gasket looked pretty dodgy, so got out the socket set and tighened it up so it was a better seal... and I did 100km's today and it seems to idle much better.
Not sure if this was the pure cause of the problems, but combined with replacing exhaust and cat, and doing that, the car seems perfect now.... I'm getting my injectors cleaned next week as well (good mainteance) and getting a heavy duty clutch whilst i'm there, so i'll get them to have a check over everything too. I'll get them to put a new gasket in there as well to prevent any furthur issues.
Damn it's nice having a car that idle's smooth again...
Thanks for all your help and ideas - without it, I would have just sent the car straight to the mechanic, but I enjoy playing around and learning.