Editing content on the 'Checkout pane'

I've been trying to find a way to insert wording that is specific to various stages in the checkout process but haven't yet found a way of being able to do this.

Example (referenced in attached screenshot):

On the 'Shipping' checkout stage I would like to add some text underneath the "Shipping" H1 tag stating "Order before 4pm Monday to Friday for same day dispatch by first class post. Orders placed at weekends will be dispatched on Monday mornings."

Posted: Aug 28, 2014
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