A little progress to report: I've started on the web interface for the controller. I can now change settings such as the target temperature, various controller options, or building parameters (for the heat loss calculation) via a web site served from the Raspberry Pi directly.
The software running in the background (which, at the moment doesn't do much other than measure and log temperatures and calculate heat loss), then picks up these settings and uses them.
Next step, getting the calculations in and running a continuous measuring and adjustment loop. I'll probably run this simulated for a few days (where the controller thinks it turns on and off the pump) to tune the math and parameters. I also want to add a page to the interface and corresponding capabilities to the backend, so I can add temperature sensors and they're auto-detected and can be named.