Ok, the plot thickens. I've worked out that whenever the front USB ports are connected to the mb the system refuses to boot. Remove cable & it boots OK. However, it now seems that all USB outputs from the original & replacement mb are now dead! I assume that the front USB has shorted and fried some components on both mb's. So it seems I'm left with 2 boards, both working OK except for no USB outputs! I could go and buy another mb/cpu but I'm thinking instead about using a PCI extension card with USB outputs. The question is though, will this still route through the mb USB circuits. For about £10-15 unit cost I suppose it's woth a punt.
Oh, how I hate PCs!