I don't get any screen, the computer does'nt even get to boot stage.

I took the power supply out (there are actually two of them!) but it
doesn't have a load of wires coming out of them like others that I have
looked at. They actually have plugs that plug into another board, so it
will not be so easy to substitute power supplies.

When I removed the power supplies and try the power again I get the
click of what sounds like a switch.

I just hope I haven't done too much damage - probably wishful thinking
though!