yep. its the white plastic "thingy" on the right side of the driver under the dash. (on RHD)

If it wont budge try getting someone to apply pressure to the front of the bonnet while you pull on it.
(worked for me)

First thing that springs to mind is a suspect head gasget

take the oil filler cap off and look inside. If its "milky" then the water is mixing with the oil also.

Are you loosing any water ?