Plugin Permissions Problem

Hi,
I have recently moved over to a cluster platform and am having problems with file permissions.

All files and directories seem to be set to 700 by default and cannot be changed because they are on a shared platform.

A problem started whereby when I am trying to install a new plugin or add media I get an error message stating :
‘Unable to create directory uploads/2015/12. Is its parent directory writable by the server?’

This has only started today.

All the guidance I can find online tells me to change the file permissions to 755 but this cannot be done in this situation. I have found that adding a new sub-domain and trying to upload works fine – but all of the existing sub-domains (and main site) have restrictions.

Install within WordPress works ok – it is just upload. I have tried multiple plugins and uploads on multiple sites and the error is consistent.

I am at the stage now where I am going to start backing up my websites, deleting them and recreating them to get around the problem.

I would rather not do this if I can help it because of the risk factor.

Has anyone had this problem before ? It’s driving me nuts – and the hosting company can’t solve it either – I’ve been in discussions with them for most of today !