In reply to Dark-Cloud:
One thing about Red Bank is that it's very sheltered under the trees and can stay pretty damp and greasy, especially at this time of year. It's also 25%. That bit of the A593 between Clappersgate and Skelwith Bridge isn't exactly bike friendly either.
Whatever problems the A591 has, cycling wise, I think it's the least worst option for a daily commute. Of course given the right conditions then the various side roads do provide a bit of variety.
Depending on just where in Ambleside the OP is living might also make a difference - if he's living down near Waterhead then the back road along the side of Loughrigg is the better route.
ALC