This sorry saga has ground to a halt. A got a new Antec Bronze 550 power supply, and connected it to the motherboard. Power on - nothing. Suspected the power button on the front panel, so shorted that -nothing. Replaced the main power cord feeding the power supply - nothing. Rechecked all header connections, still nothing. Surely I couldn't have scored a failed Gigabyte Mobo!
Is it possible that the AMD Ryzen 3 4100 4 core processor is not compatible with the Mobo? I attempted to research the compatibility, but I couldn't find that CPU mentioned. The Mobo box says not compatible with AMD Ryzen 5 3400 & AMD Ryzen 3 3200G - but not the 4100. And would that stop the BIOS booting anyway?
Any thoughts on further trouble shooting would be appreciated - otherwise I guess the whole lot will be going to land fill!