Binnan gonnan dunnit.

Finally got the header re-plumbed, albeit with a microscopic, fixable, weep; can't even, really, call it a leakSpot of "Clart" or PTFE tape'll sort it.
I found  Mk l header while setting it up, and wondered why I hadn't re-used it. Eventually the light dawned; the hole was in the bottom from original attempt. If used "here" it'd be too vertical to deliver accurately into header "pond".