Could it have something to do with the data formatting of the field in your report? It should be set as text because it's not a number, it's a string of multiple numbers.
When you select multiple options from a pop-up menu, they are stored in a single field and separated by ¶, and will be listed in the order they were selected.
Thanks for the reply, it was indeed a data formatting issue. I set the field to text and bingo it worked straight away.
Once again thanks for the reply.