Tom, Silverado blooms and increases really well for me in the south of France, although it's pretty hot there (not sure what the Zone rating would be, but we don't get frosts). We also have horribly sandy soil there, so have to add quite a lot of food, but it's so free-draining that rhizome rot is just not an issue.
I haven't tried Silverado in England, but even there I think we're probably about zone 8 (some frosts, but not very often, and it rarely snows). Do you have a sunnier spot to which you could transplant a piece, as an experiment? It would be worth the effort, because the colour (icy blue/almost white) is gorgeous and the blooms have great substance.