Thanks all. Yes, it powers on, fans whirr away, but nothing goes out to the screen and clearly XP isn't trying to start as I don't get the leds flashing on the keyboard.
I think it did try to reflash itself because the DVD light went on (and when I booted with a usb stick with the bios flash utility and bios file the light on the USB stick went on)
But no joy.
I've decided any more time trying to fix it is a waste. Unless anyone has a magic answer I'm going to buy my way out of it
Thanks for the suggestions Zing. I'm a bit wary of Asus now

oh and the mobo needs to have 4 memory sockets as I've got 4 1 gig sticks.