Data Validation drop down list (Dependant

Hi I am new to Data Validation and would like to know how to do the following.
Attached is a spread sheet which has 2 tabs, Main and Lists. I am looking to have 3 drop down boxes depending on the selections made in the pervious drop down.
First drop down is for the site I need to look at, which I think I have done correctly
Second is the area of site, which I think is also correct as you can see on the Main tab both drop downs seem to work, my problem is the next step which is to list the sample codes depending on the last two boxes as you can see from my list tab there can be different amounts of samples depending on which area you choose.
hope I have explained it correctly, can anybody help please.