This paper concerns the explicit construction of extremal Kähler metrics on total spaces of projective bundles, which have been studied in many places. We present a unified approach, motivated by the theory of hamiltonian 2-forms (as introduced and studied in previous papers in the series) but this paper is largely independent of that theory.We obtain a characterization, on a large family of projective bundles, of those 'admissible' Kähler classes (i.e., the ones compatible with the bundle structure in a way we make precise) which contain an extremal Kähler metric. In many cases, such as on geometrically ruled surfaces, every Kähler class is admissible. In particular, our results complete the classification of extremal Kähler metrics on geometrically ruled surfaces, answering several long-standing questions.We also find that our characterization agrees with a notion of K-stability for admissible Kähler classes. Our examples and nonexistence results therefore provide a fertile testing ground for the rapidly developing theory of stability for projective varieties, and we discuss some of the ramifications. In particular we obtain examples of projective varieties which are destabilized by a non-algebraic degeneration.Ψ * y = y c , Ψ * t = t, and hence Ψ * J = J c .As J c and J are integrable complex structures, Ψ extends to a U (1)-equivariant diffeomorphism of M leaving fixed any point on e 0 ∪e ∞ (since it is fibre preserving).Putω := Ψ * ω. Thenω is a Kähler form on (M, J c ) which (we claim) belongs to the same cohomology class Ω as ω. Indeed, on M 0 we havẽdd c Jc h(y c ) = Ψ * dd c J h(y) =ω − a ω a /x a , so the following implies the claim. Lemma 3. The function h(y c ) − h c (y c ) is smooth on M . Centre-ville,