The issue is not mixed data format in the source as I have checked this (I even recreated the column from scratch). Also there isn't any problem with the coding for the switch - my mail merge is to create letters that contain the date ie the date format is correct on some letters but not others.

I don't think I have explained my problem very well. Below is an example of the output of my mail merge:

10/01/2013 Tuesday, 1 October 2013 (This is incorrect)
15/01/2013 Tuesday, 15 January 2013 (This is correct)
05/03/2013 Friday, 3 May 2013 (This is incorrect)
27/03/2013 Wednesday, 27 March 2013 (This is correct)

It seems that the Word can only create the date in the format that I want when the day is greater than "12".

Surely there has to be a work around.