Warmflow leak?

I have a Warmflow Combi oil boiler, c. 8 years old, but not much used. I have drained the cent. heating system down and on re-filling it via the fill hose at the top of the boiler, water started to pour from the side outlet of a small brass pot-shaped vessel connected to pipework also located near the top of the boiler and to the left as viewed from the front. . (It lies behind and near the top of the large red Pressure Vessel on the front of the boiler). Obviously I cannot get operating pressure while this overflow/leak is happening. Is this brass pot a banjaxed pressure overflow and can it be re-set/sealed or must it be replaced? It has a very thin nut/base, and this is how it is connected to the pipework system, presumably. Any advice gratefully received.