Thanks OzExpat that's just what I was after.
That's not the whole story, however, because the protection area begins to narrow at the IF. It reaches it's narrowest point at the FAF and then narrows even further, until reaching the MAPt. Thus, the pilot must not accept the full 2.5 NM off track in the Initial Segment because the aircraft will end up outside the procedure's protection area some time after passing the IF.
I suppose the issue at the heart of this is the proximity to track required outbound before commencing descent to the base turn level.
One way of looking at it is that even if 2.5 nm off track at the end of the outbound leg at the altitude for the base turn, the pilot can still make the base turn and establish within the tighter tolerances required beyound the IF before further descent inbound.
I suppose one could envisage a situation where an obstacle beyond the IF is far enough away to be outside that primary area for the intermediate segment but within 2.5 nm and high enough to be an issue even at base turn level.