maka2k2 the fact that you are trying to help is great but as you are, by your own admision, unqualified and inexperienced it might not be a bad idea if you slowed down and thought sometimes before you post.
Please dont take this the wrong way but you have given a few really poor tips and you...
Why dont you undo the front of the overflow then drop off the whole thing.
Drop a piece of string though and use that to pull a new overflow up.
Refit cover and Robs your mothers brother.
You be careful you might need a priest.
Ooooooooooooo
LOL
People amaze me " do I need a plumber?"
Maybe maybe not but you need a person to look in your loft more than you need to sit at the PC asking daft questions.
LOL
Unless the pipes and rads look gash you are being had over. The system has to be flushed cos its the rules so sludge is not an issue, in addition the warantee is carried by the manufacturer not the installer. Wake up smell the beans and phone a few more.