Hi alba
http://www.tesco.com/groceries/Product/Details/?id=251590774
Try some of this stuff at regular intervals ( maybe once every couple of months ).......pour the whole lot down the plughole.....leave for a while, about 10-15 mins i'd say, but at least 5 minutes, to work through....then pour down some hot running water from the tap to clear.......presto....hopefully blockage sorted.
Next time young lady, I wouldnt put a pair of your frilly knickers down there, eh?.
Hope you get it sorted, alba, my little bakewell slice. :0) xx