... The worst aspect of AV is that it dramatically reduces the chances of a single party winning outright a general election.

Again, this is a largely erroneous view.  Experience where it is already used indicates that whilst it can reduce the number of seats that any single party can win, the overall result is that you get smaller majorities rather than no majorities, requiring a single party in power to work with other parties on specific bills, which would - generally - result in fewer examples of 'extremist' legislation.