How can I stop the overflow on my heating system?

I have a Potterton Suprima 60 boiler. Had a replacement part fitted about 2 yrs ago, the engineer said that the motherboard? could not be repaired and it was a fault with this type of boiler. He fitted an earlier version. A week ago the pressure dropped to 0.5 so I put water into it via the tap. Now the system overflow will not turn off and is currently cascading down the outside wall. Can you explain the problem and tell me how to repair it.

Thank you. Pauline