Many thanks for your reply Phil. I've had a go at changing the fonts and the fonts selection as per your instructions but this doesn't bring up anything in the new entries (they remain completely blank), it just changes those that were working ok.
I've got a screenshot to show you what I mean about the barcode suddenly not working
As you can see, it suddenly stops working where we have added new entries. The calculation for the field is the same throughout which is why it's so puzzling. If it helps, the instructions for the relevant fields are:
I'm not sure if this sheds any more light on the problem?