This isn't the problem I keep reading about with UK to US date format but it must be linked. In my case the actual date of a Word letter (nothing to do with any dates in the Excel data source) is changing from 7 February 2014 to 2 July 2014 when I link to the data source.

The date is driven by a Word field:
{ DOCVARIABLE LetterDate \@ "d MMMM yyyy" \* CHARFORMAT }
We use this in our standard template and the field reads from the document variable LetterDate that is input via a dialog box. I've had a look (using VBA) directly at this variable before and after doing the link via mailmerge and it doesn't change from the initial value "7 2 2014". What must be happening is that the act of linking to the Excel data source is affecting the way the variable is being interpreted by the field: before the merge it interprets it UK-style and afterwards moves to US-style. We have just moved from Word 2003 to 2010 and this was not a problem in 2003.

I found the following link:
http://support.microsoft.com/kb/320473
and method 1 works - although this is obviously a different problem as there are no dates in the Excel data source or any other formatting there that is going wrong, just the existing date field already in Word.

I will now need to transmit this to the many users of this template. Can anyone please suggest an easier fix? These are people who are not used to navigating Word 2010's labyrinthine File/Options...