Magento observer event when user has submitted their billing details

I am trying to find a Magento frontend observer event that will trigger on the 'onepage checkout' page when the user has entered their 'Billing' details and clicked the submit button.

I am using Magento 1.7 and have come across this site which has a list of events, however I am unsure which this would be? http://www.nicksays.co.uk/magento-events-cheat-sheet-1-7/

Can anyone suggest anything or point me in the right direction?

Solution

The following worked observer worked:

checkout_controller_onepage_save_shipping_method

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