How to: access the Honeywell Evohome API documentation
Since writing this post, Honeywell has sold their products to Resideo, the new URL is: https://tccna.resideo.com/WebApi/Help/ApiIntroduction
After my last post, I did some more digging. I found that the API is actually documented at https://tccna.honeywell.com/WebApi/Help/ApiIntroduction
you need an APP ID to log in with (or OAuth app ID+secret).