hi all i have a octavia vrs just decat it and now it over boosts can i wind the atuatior down to stop it for now 

no, don't touch the actuator. If standard it will still be using an electronic boost controller which sends a signal to the actuator so unless it is a sticking actuator, fiddling with it won't help.

Only thing I can think of is that the wastegate or actuator may have been damaged preventing it from opening? Who fit the downpipe?

a friend of mine car was running fine been told as it was mapped with cat and now no cat this is why its doing it only on full load 

It's possible but seems unlikely.

Anyway to answer your question, the electronic boost controller controls the opening of the actuator so the best way to solve it is probably to get it remapped which will change the required boost.. that's if it was just the decat causing it.. (I can't really see how though) Is it a full bigger exhaust system too?

How do you know it overboost, is it throwing the car into limp mode