just to recap' computer goes to S3 sleep OK, power light blinks and monitor goes to standby.
Press any key to wake machine OK. Power light comes on steady, fans start, hard drive light flickering (suggests activity), monitor stays on standby (black screen) or says no signal. Reset button now not responding. Have to power off machine on main switch.
This is now a dual boot computer, have installed XP and that goes to standby and recovers OK. Perhaps suggesting that the BIOS settings are OK.
ASRock main board has the new UEFI bios that I have little knowledge of, so I may well be using incorrect settings.
Thanks to all who have helped, TNT.