i have problems with detanating at 18psi cav turbo 4x4 were bad for it so my mate with a corsa with same engine had same prob 300bhp running at 20psi same prob detanating he put volvo t5 plugs in it £55 will get u 4.now im running 15psi at 285mph it nice to boot it and not splutter.
CallyT - even the recommended GM plugs are pretty good. They often get this kind of stuff straight from Bosch.
If you are running standard boost, stick with the recommended plug, GM or better still, get good Bosch ones (usually cheaper).
Also, Gav at Wallaceperformance will get you the best prices on anything, but I have a chargecooler kit for sale, it is VGC, but needs the little radiator replacing, about £130 from Pace I think.
Cavyturbo - you don't need to pay £55 for plugs!! My car peaks at 22psi on overboost, and holds 17psi, the best plug to handle this is a Bosch F6 DTC (£3 to £4 each!). Essential to get plugs in the right heat range (usually one or two points higher than the standard) when you are running higher boost. Otherwise they can't expel enough heat, and you will get the detonation problems you mention. Hope you didn't let it det for too long, once the damage starts...........
Also hope you replaced the knock sensor straight away. It "usually" does a very good (and IMPORTANT) job!
Richie is right (as usual), get that charge cooled, if you haven't already.
Are you sure you are running that much BHP? Without any form of intake charge cooling, it is really unlikely.