Right. Question:
Can you use an 11sp Ultegra mech on an other wise 10 speed drivetrain?

I’ve had a good ferret around the interweb and as far as I can tell, it won’t work, but there’s very little info around on this as yet. Also, the bike in front of me tells me it wont work too, as (With no chain fitted) it looks like it’s simply not pulling the mech far enough per shift.
I’m assuming the mech geometry is different as well as the cable pull at the shifter
Am I right or wrong? 🙂

I’m guessing you can’t. As you’ve proved to yourself already. Twice 😉

Nope, 11 speed rear mech is not compatible with 10 speed system. It’s all on page 241 of the 2014 Shimano Trade Sales And Support Manual.

Superb. Thanks Mister P. 🙂