
The best thing I've found for unblocking pipes is, believe it or not, cheap shampoo! You need to squirt quite a lot down (1/4 of a bottle or so), which is why cheap is best, think Tesco Value rather than your best Loreal :) I'm not sure how you'd get it down an overflow as it needs to be squirted, perhaps some sort of pipe on the end of the bottle?
Follow it up with a couple of tsp of bicarb of soda and a fgood flush through with boiling water a few minutes later and hey presto! Clean pipes :)
Claire