How can i fix my Vokera Linea boiler?

The boiler overflow pipe that goes outside is dripping hot water frequently. The pressure is also very high at times. The water & heating are approx. half way up for heat setting, so not very high.

Please could anyone answer - Why is the pressure high & how can i stop the water leaking out? it has been happening for sometime as the lower wall is greeny /mossy/ damp where the warm falls.

How can i fix it?