I'm in the process of doing some fairly basic XML to PDF stuff, and I'm getting this exception, and I'm not sure why.

Which as far as I can tell, should work - all my blocks are in flows, in a page-sequence ?

Read the message again: it means the flow contains stuff which
is not a block-level FO.
In general, a good attempt at debugging is to run the XSL
transformation standalone in order to get a FO document and
take a look at it.