Thanks for the reply silkrooster - it's definitely a permissions issue, so thanks for pointing me in that direction.
However, I'm still a bit confused - for some reason, read access isn't sufficient for this. After a bit of trial and error, it appears that I can only get error free access when permissions on the directory are set to 777! This seems a tad..erm..excessive!
For example, currently the situation is this:
and I have no access to the files over my browser. The ftp programme can access the folder, but only after I'd enabled writing for group users (for some reason, 764 is not enough).
Chmod 777 however makes the files readable as you would expect.
So in a sense, it's fixed, but I would love to know why the permissions have to be so extensive, when you would think a web browser only needs read access to a file/folder to display it...any ideas?