However, one serious weakness enables supporters of the majority party to select the Opposition in the Commons as well as the Government. The right to vote for one party in the constituency section and another in the top-up section allows widespread opportunities for tactical voting.
For example, in West Yorkshire Labour are certain to win virtually all the constituencies, and cannot therefore win the top-up seat. Labour voters may cast their second vote for the Liberal Democrats to keep out a Conservative top-up, instead of wasting the vote on Labour. Such votes would swamp the Conservative top-up votes, leaving the larger minority unrepresented. Of course, in Tory heartlands a similar process would exclude a Labour top-up. No wonder the Liberal Democrats are so keen on this system!
Instead of the second vote, the ballot paper should include the question: "Do you wish to support the party of your first-preference vote for the purpose of electing a regional top-up MP?" This would preserve the right of voters to support an individual, but not their party. It would also ensure that the top-up actually goes to the most under-represented party.
Your correspondent Richard Bartle rightly suggests that the top-up MP should be the best runner up from the appropriate party. Thus the successful candidate is chosen by the voters, not the party machine.