There isn't an API, I scrape the website. From time to time they change the website in ways that break my poor code. It's currently ~100 lines of Python including lots of debug for when it goes wrong.
Nice. How often do you use the lido btw? I am there most mornings now and will be throughout the year, alternating with the ponds as I train for the Dart 10K
Oh BTW I wanted to ask, is this your doing?
http://www.jellybaby.net/ponds.html
If so I love you and do you compile the data yourself or is there an API (I doubt that somehow) :D