thanks guys all very confusing to a relative novice! I've bitten the bullet on a extender to see if that cures things, I'm loathed to buy another mech nor a cassette given the cost! I like my single chain rings so I feel I really need the 11-40 range at the back...
It's annoying as I have an xt 11 speed sitting around brand new but my wheels hub won't take an 11 speed cassette!