OK I've worked out most of the problem: permissions and ownership of course!
/usr/ports/distfiles must be owned by _pfetch:_pfetch
/usr/ports/logs /usr/ports/packages /usr/ports/plist /usr/ports/pobj
need to be owned by _pbuild:_pbuild .
Also, if you're signing with your own key, it must also be owned by
_pbuild:_pbuild .
Once that's all sorted out building works as root with dpb subdir/port.
However I can't seem to make it use my list of ports to build, it just
complains about a bad pkgpath. This functionality worked in 5.9, doesn't
anymore, which is rather annoying.
Cheers,
Noth