How do I fix my Main Combi 24E Boiler leak?

Boiler is 3 yrs old, has been reliable and continues to work but I have a steady drip leak from immediately behind and below the Pressure Differential Valve. Hope I'm right about this component, it looks like a housing for a diaphragm as it is a metal case fastened with 6 screws. The leak is coming from what looks like a drill-hole and I'm wondering if there should be a screw in place there. Is it something I can do myself? All advice much appreciated.