Ok, I've tested the RAM in another machine, and it's fine. I discovered (to my disgust) that I had put one of the motherboard stand-offs in the wrong spot (under the com port) and that it had bent the underneath solder joints a bit, but nothing was broken or touching anything else. I removed it and tried it all again, with the same result.
There is another thing that I am wary about... there is a jumper to reset the cmos, that I left in the same position as what it was when I got it. On the board itself there is a white arrow pointing to the right hand pin, so I assumed that was pin 1. In the book it says that the jumper over pin 1 and 2 is the normal setting, so I thought it was fine. Then I looked in the book, and by the look of the diagram, pin 1 is actually on the left hand side... so I tried it the other way around, but it didn't make a difference.
I'm running out of things to try, and I'm beginning to think I've stuffed the thing completely.
It gives one beep when you boot it, as if to say everything's fine, then it detects the cpu, and that's all...
Any more help would be greatly appreciated