I would like to share a solution to a problem we were having in case it helps someone else. I have Barry Dickens to thank for this.
As displayed in the image below, if we sent a print job to our B&W print queue, but the source (e.g. web page) contained colour, we would still have a reference to "color sheet/s" (e.g. "1 color sheet at a cost of $0.25").
This would still occur even though the "Printer Type" for the relevant "Device" was set to "B&W" and the "device type" in "Printer Properties" in Windows was set to "Monochrome".
The fix in the end was changing our driver for the Black and White printer from "HP Universal Printing PCL 6" to "HP Universal Printing PCL 5". We have kept "HP Universal Printing PCL 6" for our colour printer.