ok pulled out the front panel cables and it works now. in my defence the little cable connectors use very different terminology than the motherboard manual uses. Lets never speak of this again
/request "numpty of the month on hexus" award
not convinced we're out of the woods tho as the only USB device I have attached just now (and it wasn't even on one of the possibly incorrectly wired ports) is an old Dell optical mouse. but it certainly POSTed OK with that mouse attached when it wouldn't before.
Thanks all for the input!