Last 12 month's average

Hello,

I am trying to create a formula at the end of a row that will give an average for the past 12 months data.

Simple enough I thought, however I am having difficulty in making the formula change each time I add a new month.

I would like to add an extra month column to the sheet and have the average include only the new month and the previous 11 without having to manually change it.

Attached is a document showing some of the data which will hopefully help in resolving my problem. I am using Excel 2013

Any help would be great, thanks