Quick update.
Fitted the Shimano Hollowtech 2 cranks today, together with the prototype rear derailleur hanger with derailleur attached and threaded on the chain. The hanger position has worked well as planned for the positioning of the derailleur while setting the correct horizontal measurement of it relative to the rear drop out. After setting the two limit screws for the high and low gear I moved the derailleur by hand and it reaches the 42t cog no problem with a little to spare. I then set the B screw and this easily positioned the derailleur within the 5mm - 6mm tolerance required in the derailleur set up instructions with plenty of tolerance on the B screw either way so well chuffed.
I then decided to try fitting the chain and this worked well although I will need to add a couple of links to it when I properly fitted. Also I noticed shortly after fitting it that I had threaded the chain incorrectly through the derailleur. Just shows it pays to read the instructions first...
Photos of cranks below...