I'd want them made out of gold, that's more than £10 a plug! You can get genuine GM ones either mail order on Ebay or even via AndyC, so there's no excuse for not buying genuine

On a serious note mate - if this issue has started since the introduction of these plugs, and was fine before, then it's quite a co-incidence if it's not related. I've heard good things about Denso plugs, in fairness to them, and they SHOULD be absolutely fine - BUT they are still an unknown quantity in this equation. I would say you need to try it with genuine GM plugs and take it from there. They just might, have some sort of incompatibility.
I think I speak for everyone who has contributed to this thread so far, in saying the advice given is to put genuine GM plugs back in there and road test.
At the end of the day GM plugs are a couple of quid each, so even if it turns out you didn't need them, you'll have them in the glovebox/boot when the next service is due. Or handy spares.
What you describe as a "pinking" I think you're actually trying to explain the symptoms of a HT misfire - which, by very nature, will be much more apparant under engine load - and not log any fault codes in the ECU, as it's a non DBW engine. Therefore it would also be very wise to individually check each HT lead, ensure they they are not split/cracked/damaged, are fully home, so on so forth. Pay specific attention to the shaft of each lead that goes into the plug hole.
Also, without wanting to sound like I'm teaching anyone to suck eggs, were the plugs properly tightened? If not they can work loose very quickly...