My n900 display has gone!! Can any one tell me the correct ribbon that I should replace?

well, there's only one that can be replaced if I remeber correctly. The ones from LCD and digitizer cannot be replaced.
Maybe it's not the main ribbon cable but LCD itself.

You can check which ribbon has truly been damaged-cut-destroyed-ripped-or-snapped by partly disassembling the device. You will need to do it, either way, to replace the ribbon. Resources. Just, don't damage the screws like I did - in the worst case, I used glue in their place.
Most likely, it's main flex ribbon - it's the most exposed one. Other ribbons, as kinggo says, are inseparable from their hardware (such as LCD, digitiser) - and since they are inside the device, it's quite difficult to damage them.
Examples of places where you can buy a new main flex ribbon cable are mentioned in the above-linked threads.
Best wishes.