Miliband only seems to be going because he's been told to by Kouchner. It doesn't sound like there will be any more troops sent in by the UN although a Foreign Office minister called Mark Malloch-Brown said that the EU would not 'stand back and watch violence erupt'.
The reason the 17,000 UN troops already there are so ineffectual is that they are made up of 'inferior' armies. They have no experience with peacekeeping (unlike the US and UK forces) and have been accused of child abuse and corruption while there. So they have lost the faith of the Congolese army and people and they are actually refusing to take on the rebel forces. Supposedly the Spanish commander (highly respected) quit after 7 weeks of command because he was so pissed off with them.