Ravenheat RSF 820/20 - leaky part identified, but what is it called?

Inside the Ravenheat RSF 820/20, on the left hand side, at the back there is a roughly spherical part that is attached to the cold water inlet of the boiler proper. It is leaking. Can anyone tell me what it is called so I can get a new one? (and a recommendation as to where I could get such a part would be doubly appreciated)

I'm guessing it is some sort of pressure vessel - maybe to smooth out shocks caused by valves opening and closing?

As it is only attached to the water supply I feel confident about replacing it myself. However, if anyone has just sucked air through their teeth and thought "he's got no chance", I'd be keen to hear why. I am prepared for the fact that my knuckles might take a beating.

Ta