Hi Brian, many thanks for the message and the hint.
I've looked into the code again and you are correct, it hadn't retained the 'Default File Name' entered in the 'WA UploadResult (updatebriefpic1)' for the 'brief_pic_1_full' image. It looks like it is the recurring issue of not hitting the return key before pressing 'FINISH' in the pop up window. I've tried again, remembering to hit the return key, and this time it has worked perfectly.
Thank you again.