The water that comes out of the Condense drain will be dirty as it is generated by the Combustion process, if your system was powerflushed 3 years ago, the water should have been treated with inhibitor on completion, so further power flushing should not be required, if all the radiators heat properly this really confirms this.
Make sure your Service Engineer understands Condensing Boilers especially the need to clean the Trap in the Condense pipe
www.elanar.co.uk (Grant Service Agent), May 2009