sorry to feed users…
I’d really decided I wanted to just use the default p=n style of permalinking. I had hoped permalink schemes could run in parallel, but it seems not. In the interests of preserving what’s already out there I have now reverted to “Date and name based” to maintain compatibility.
(update: actually ?p=nn style permalinks are still ok)
Hopefully this is an end to the tweaking required following the migration. If anyone is having trouble with this blog, I invite comments here.