How can I fix a water leak on my Ford Escore P Reg?

I recently noticed that the water bottle in my P reg Escort was almost empty. Took it to a garage and they told me the water bottle was leaking, charged me £80 to replace but since then the same problem has occurred and is ongoing. They told me that they could not find any other leak but the leak is still there. What could be the other possible cause for the water leak.

Gene
January 2010