Duncan, the source became closed after a company that manufactured a wideband system bought a v3 from us, and about 6months later came out with a range of lower cost, lower power consumption wideband products. We became a little suspicious that all was not as it should be when we noticed that they had fresh air calibration and a few other quirks that looked familiar...
So to ensure that nothing like that happened again (as a number of advanced features were being planned, and now that there are five people working full-time in hungary), the source and hardware was closed.
You can still get access to certain modules if you want to apply for it, the general idea is that you identify the issue you wish to resolve or the feature your wanting to add and then you can get access to the particular source files and the objects to compile against.
Because there are developers working full-time on this its often better just to put in feature requests and wait for a few releases.
I'd be interested in what developments you'd be looking to do.