I have a renault lagina 1.8RN with the same prob (very common), I did the same as Alison but it was being stuban so in my annoyance (and unwilling to give any more money to renault) I brought some powerful drain cleaner (made sure it was O.K for Metals) and sat the matrix upright in the sink and kept pouring some in let it soak over night then used the pipe from the shower (for pressure) to flush it, took a few times but its the cleanest it ever been.
ste, February 2005