Check out the LS Focus fault register thread. It seems its a common prob which isn't fixable.
We had the leak on the drivers side of the hatch. Its fixed now (at least it seems to be) after 4 attempts at it. The leak is in a join in the body up near the roof. Its very common fault in the LS. They managed to seal it from the inside in some way. The forman at Bryan Byrt Ford in Capalaba is a Sth African guy with BMW and Mercedes experience he used a few neat tricks of the trade to locate the leak. More than I can say for the body shop that had it twice and did naf all :(