Actually, don't worry! I have just realised that I am meant to be looking at line 213 of resize_image.php rather than the code on the home page.
213 of resize_image.php refers to permissions and then the penny dropped. Because I have downloaded the whole size to collect everything including amends and new images byt the client, I have downloaded a folder that is locked... I just needed to change permissions at root level and apply to all enclosed items so the script could create the files needed automatically as you said.
Just thought I'd highlight this for anyone who has the same problem!