problems with file permissions and IMCE in sites/default/files directory

I have looked around a great deal on the Drupal forum and elsewhere but I cannot yet resolve this.

I have had to reinstall a large, fully functional site (D7.18) onto a new server. This has gone very smoothly. However, I do not seem to be able to set permissions for my sites/default/files directory in a manner that keeps it accessible and safe when browsing using IMCE.

• Usually I set sites/default/files (and subdirectories within it) as 755, with files within these directories as 664. This works well on many other Drupal 7 sites I have built.
• HOWEVER in this case, with these permissions I get the message "Unable to get a working directory for the file browser".

Only by setting directory permissions as 777 can I browse the files in these directories using IMCE - and I know that is really BAD practice on shared hosting.

Please can someone advise on troubleshooting this? I have spent hours but I am getting nowhere.

I wonder if the ownership of the files and directories themselves is wrong. If they are wrong, can anyone direct me to step-by-step instructions for changing them?

• Examining the 'problem' files and directories using FireFTP, I see that both user and group names are the same as the FTP username that was given me by my web host.
• Looking at another Drupal site that works properly, I see that files and directories in sites/default/files are set to user 531/group 528.

Thanks in anticipation! I am running D7.18 on PHP 5.2.10 with extensions enabled. Everything else seems to be working very well indeed.

NB - is it possible that things would work better on PHP 5.3? I think this site was previously on a PHP 5.3 server.