Finally resolved the problem (with help from the IT dept. at work)
Previous copies were mpg1 format which IT had installed Indeo 5.1 coedc to allow me to play the files in PowerPoint. The latest versions were mpg2 format which requires some sort of a DVD codec (which the company were not prepared to buy!).
Re-encoded as mpg1 and all works fine.
The posting is just for anyone who may have a similar problem at some time