Just bought an N900,and I'm super excited for it. I got it second hand on Gumtree.

I was really keen when it arrived, but sadly I can't get it to turn on.Ive plugged it into my computer using the cable I was given, and sadly I get nothing from it - no power light or anything, no response from the computer (as far as it's concerned there's nothing plugged in) The only indication that any power is moving anywhere is that it makes a faint repeated noise (I can't explain it well, but it's like a hum or a hiss and then nothing, then sound, and nothing etc...) I've tried leaving it to charge and I still can't get the bugger to work.

I've looking into trying to flash it, but sadly it seems the site where you download the files from is dead.

guessing the battery is flat?

try hooking it up to a wall charger ( the original if possible ), there should be an orange light come on in the bottom left corner, leave it to charge for a while

the n900 from memory supports an older "dumb charger" standard so you may need to try a few different ones if you do not have the original charger

steady orange - recovery charging, the battery is to flat to "boot into normal charging mode"
blinking orange - normal charging
steady green - charged

Have you tried the repos on the wiki ?

I had one similar to this, I guess it had been sat around for while, bought a new battery and she was up and running again!

My spare n900 after almost 2 years in the box behaved in similar way. When connected to original Nokia charger (that was in the same box as the device), even the orange light did not blink. I leaved it connected for some time and after few hours (!!!) - maybe 2, maybe 3 - it started charging.... No problems since then. So, as suggested earlier, maybe try connecting it to the charger and leave it like that for few hours...

Update: I've left it charging in the wall for a while and so on and noticed that it displays a solid green light (Yay! It's charged, right?) but oddly it's only on when I touch the edges of the phone. I'm guessing this means that the charging port has a dodgy connection and it touching the exterior/not touching the internal circuits, which would suck. I've left it on charge overnight to see if it improves.

If the phone is unresponsive even though the battery should be charged, take out the battery, hold down the power button for a few seconds and replace the battery.

You should find pretty easily posts about the N900 & broken micro-USB ports and possible fixes. What I've come to learn is that it never hurts to file down the little metal hooks on the cable as a precaution.

Is this the tabs on the cable? (as seen in this picture:

or on the phone, in the usb port?

Exactly those, on the cable.