Sime Format 80 C?

Hello, My Mothers Sime boiler has started to leak, the leak is just above the Cold Water Inlet for the Direct Hot Water supply. Above the quarter turn shut off valve is a black part (I think it is the Divertor valve)? On the black part (I know technical) is a black nut which will take a cross head screw driver, there is where the leak is! I've tried to tighten but the nut will not bite and just keeps going round? Has any body got any idea on what I could do to fix it? Please don't say get a plumber as I am one already but am a bit unsure on this perticular boiler. Many thanks Lee