Well I soldered the Tri State to the Ground following various pictures on the forums and this didn't resolve my problem. I still have the same symptoms !!
Press the standby button and 3 seconds later the Slim turns itself off with no lights. If I press the Power button the standby red light goes back on.
I have looked at the motherboard and there are various fuses (labelled Fxxx) but they look ok and they are far to small to solder a new one if they weren't. I tested the big fuse in the power pack with a multimeter and thats ok.
Any ideas what to try now? I read somewhere that you can get the PS3 some sort of diagnostic after the fan test (Fan test works by the way).