PJ - the only comment I have is my surprise when I was firmly informed by a couple of BA guys that in a lot of cases 'breaks' are rostered and mostly not interfered with.
BOAC,
You may have got the wrong end of the stick. Most long haul flights carry three pilots - ie 2 FOs, and 1 captain. To that end, one of the FOs will be rostered "heavy" on each flight. This is not set in stone, and is negotiable, if for some reason, someone wanted to swap on the day (tired/commuter etc.).
The role of the "heavy" FO is to do any odds and sods the operating crew want him to do (walkarounds/security checks), and then go to sleep for the first leg of the journey. He then replaces each of the operating pilots, in each of their seats, whilst they sleep, and retires, at top of descent.
The "rostered" breaks are done on the day, so not set in stone, either, although all will be given a perusal, to confirm that they are fair.
This whole "the captain would never leave 2 FOs in charge in the ITCZ", is frankly, bollocks. Why not? They've all got ATPLs, and thousands of hours, and the captain needs his rest. If they're not capable of using a weather radar set, and avoiding thunderstorms, then frankly, they shouldn't be there. But the default position is that they can.