European elections - how does it all work

EU elections are now almost certain to happen.

They use a version of Proportional Representation known as the D'Hondt Method.
I won't try and re-explain it here, as there are already various good explanations available such as this one.

I've made a google spreadsheet partly based on some other ones I found online, to let people play around with different voting patterns and see how it affects the results.