Any question answered...

Will the Dura ace 9000 BB work with 5700 cranks. If so, is it actually worth the additional £11 cash that it costs compared to the 5700 BB on the Merlin website.

1. Yes
   
2. No (unless you're a weight weenie, in which case it's pretty good value in £spent:gram-saved)
   
   

In a classic Shimano dick move, they have changed the BB cup tool interface for 9000 & 6800 (and presumably 5800 will follow). The TL-FC24 plastic adapter is obviously up to the task of installation, but it's an open question whether it will survive removal of the cups at the end of their life, assuming you haven't lost it in the mean time. Also, if you can't fit a proper socket type tool to the adapter, you have to guess the installation torque rather than using a socket attached to your big torque wrench.

The TL-FC34 which you're going to have to buy is £22.49, presumably there will be Park etc. versions in the fullness of time.