21cm is a long way out from the wall. A bracket will create a lot of pivotal pressure on the fixings at this length, but if you have no choice: you could use L shaped shelf brackets at each end and sit the pole on top of them securing with a screw. The curtains will be hiding the end brackets anyway so it does not matter if they do not match. The centre is the problem. A company called Byron & Byron make long support brackets but they do not come cheap! I have overcome this before by buying a length of pole the same diameter as the wooden brackets and staining the same colour, cutting the correct length of the extension and then securing this extremely well to the wall. Then it is a simple matter of pilot drilling wood screws into this extension for the original brackets to fit to.
Steve
October 2010
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix curtains.