Manually flushing each rad is ok and will help get rid of loose debris and sludge, but you cant beat a full powerflush which cleans all the pipework etc of the complete central heating system, you can get a considerable build up of sludge and debris in the pipework, but this does depend on the overall age of the system and if its been treated with inhibitor etc. With a powerflush you can clean all of the system in both directions as long as you have bi-flow radiator valves. If you decided to go for a powerflush its worth getting numerous quotes as prices do vary (try avoiding British Gas as they are the dearest) Hope this helps?
Marko
July 2010