Thank you Paul for looking deeping into a potential problem. What you have said makes perfect sense, and I have been aware of the problems that can arise out of using mixed data type in a specific column. I was careful to ensure that only numbers were in number columns, and text in text column. Those fields with the "<" or ">" I knew were text. These fields are displaying properly.
It's the 4 columns that are purely numbers, that aren't displaying properly in my application ... columns N, O, DG, DH, DI and DJ. Is that theory still valid considering there are no mixed data types in those columns ...?
I've attached some pictures of the mailmerge document during result previwe, and then the final document. Perhaps there is something wrong exclusive to my end.