Re-Build went surprisingly easily.
Several factors:
Advice here to connect cables before fitting motherboard.
New Seasonic PSU is "fully modular", new to me, the last one I bought had the two main power cables fixed, with the new one I connected the cables and got on with things while the PSU was still in its box.
A "Q-Connector", made for the ASUS 170 boards but hopefully available for others. Ironically, like USB connectors, you have to make several attempts to get the plug the right way round, currently the power switch is fine but no power or LED light so I'll turn it round at next power off.
formatting link
Booted first time, one of the feet on the case is broken so I need to order some rubber door stops which make excellent replacements (that may be a tip from in here as well).Good confidence builder for me, thanks for the help :-)