The green LED is telling you that standby power is there. To test the PSU further you would have to rig up a dummy load because most ATX power supplies will not start up without some load. Have you checked the power switch. The contacts should be closed while the switch is depressed. Have you tried resetting the processor CMOS, and checked that the reset link isn't left in the reset position.
System is now operating "normally" apart from the fact that it isn't recognising the CD-ROm (as primary slave). I shall leave it for now as I'm away for the next couple of days and tackle it when I come back. Many thanks to everyone for your help.