If a document was printed from Microsoft Word it was fine; if a GP report as printed to XPS and then sent to the printer from there then it was also fine. It was just the reports printed directly from Microsoft Dynamics GP which had the overlapping text problem.
Doing some more testing I was able to narrow the problem down further, from thinking it was all GP reports, to only the text based reports. Not having seen this problem before I thought a quick online search might find me an answer.
By using the information in David’s post to update the settings in the dex.ini file I was able to resolve the problem.
I added the following lines to the dex.ini file on each client (and left notes with the client’s IT department so they could do this on any new installations):
FontCourier New=Lucida Console