I've worked out where I was going wrong. The target geometry in darktable had defaulted...
So, I dl-ed and built lensfun 0.3.2, and linked it to darktable, and ran lensfun-update-data...
Ok, I'll try it when I get home later.
Ahhhhhhh, I removed it because I was using darktable (2.0.6 I think) and it complained...
I tried out the submitted poly3 model: <distortion model="poly3" focal="3" k1="0.01419"/>...
fwiw, here are my results: <camera> <maker>GITUP</maker> <model>GIT2</model> <mount>gitupGit2</mount>...
New lens: gitup git2
I'm waiting for this too.