Longest possible single-fare journey in London Fare Zone 2?

I've noticed that it is theoretically possible to perform an almost complete circuit of London by rail without leaving fare zone 2 and without having to swipe in / swipe out with an Oyster card multiple times, as follows:

1. Whitechapel to New Cross (Overground)
2. New Cross to Lewisham (National Rail)
3. Lewisham to Peckham Rye (National Rail)
4. Peckham Rye to Battersea Park (National Rail)
5. Battersea Park to Clapham Junction (National Rail)
6. Clapham Junction to Canonbury (Overground)
7. Canonbury to Hoxton (Overground)

However, I found some time ago that when I travelled along legs 5-6, I was charged a zone 1 fare, even though the route I took did not pass through zone 1. Leaving aside for a moment the question of why anyone would want to follow this route, how can I check what the appropriate fare is for each leg? And just how far around zone 2 can one travel without being stung with a zone 1 fare?

This map may be useful for visualising the route: http://www.tfl.gov.uk/assets/downloads/london-rail-and-tube-services-map.pdf

Update:

After reading @Ray's answer, I remembered that the section for which I was charged a higher than expected fare was actually from Battersea Park, not Clapham Junction. As of 9th March 2012, the TFL fare finder currently suggests an off-peak fare of £3.10 for this journey. The national rail through fare list has a matching entry for a zones 1-2 journey.

Interesting note: according to the fare finder it's 30p cheaper overall if you split the journey at Clapham Junction, as each leg is then counted as a zone 2 only journey, and the national-rail-only fare is cheaper than the equivalent London Overground fare.