I hadn't realised that 2.4 GHz 11n AP's were not allowed to start up in
40MHz mode.

"Reminder: Wi-Fi Requirements update

The 20/40 Coexistence test new requirements
If the DUT can do 40 MHz BW in the 2.4 GHz band, then coexistence is
required. If there is no coexistence supported, then the DUT does not
respond properly when there are devices on the extension channel.

The CERTIFIED n test plan v2.0.4 is not explicit in this requirement,
and now have updated on test plan v2.0.5 to reflect that coexistence is
required for 2.4 GHz devices supporting 40 MHz BW. The MRD covers this.

In the 2.4 GHz band, the AP must back off to 20 MHz when it senses the
following
‧APUT is not starting a 40 MHz BSS in presence of an 802.11g BSS.
‧APUT is appropriately switching from 40 MHz to 20 MHz in presence of 40
MHz intolerant STA.
‧APUT is appropriately switching from 40 MHz to 20 MHz when receiving
frames disallowing the use of 40 MHz channel width.
In addition, an AP must not power up in 40 MHz mode in 2.4 GHz band."

"http://www.adt.com.tw/english/news_files/BVADTflash/BVADTflash046.htm#bookmark14"