I made this site because I couldn't find an up-to-date source for council compositions with accessible data. I'm aware of Local Councils, Keith Edkins' Site and the Vote UK Forum, all of which are fantastic resources, but none meets the requirements of being up-to-date and providing an accessible data format.
I've made the site and the CSV databases (download links above) Open Data, using a Creative Commons license.
I named the site Open Council Data UK without particularly thinking about it, which naturally compelled me to cover all English, Welsh, Scottish and Northern Irish councils, as well the Corporation of the City of London and the Council of the Isles of Scilly.
Lots of information sources on councils will use the term "No Overall Control". I consider this to be journalistic code for either "Not yet decided" or "We couldn't be bothered to check". In reality many councils are run by a coalition of parties, so although I've kept references to NOC where there's no majority, I've also added the coalition in control (if I can be bothered to check, of course).
Warning: the 'next election' data is not yet complete for Metropollitan and Unitary councils which elect by halves or thirds. For now, every ward is listed as the next election date, whether that's 2018 or 2019. I'll get to this soon.
If you find this site useful or have ideas for how it could be used for the public good, my email link is at the top.