an update.
I was planning to use xtr 10 speed rear mech, which came with the bike, with 11 speed shifter but it needed the b-stop to be wound out fully plus a small spacer so the cage clears the 42t sprocket.... and that is before adding the spacer to increase the arm length.
Unfortunately this results in the mech being put further back with the chain engaging 2 teeth less when compared to pukka 11 speed mech. Rather than risk any chain slippage, I've ditched the 10 speed rear mech for 11 speed.