the white thing is a expansion vessel, there is 2 posible resons y this is leaking. 1 is the rubber or fiber wosher has perished between the hose and vessel or the vessel is fucked and over presurising and leaking, is the discharge pipe 2 out side blowing off?
to be trueh full, 99% its just the wosher, go 2 b and q and get a 3/4 fiber or rubber wosher and swop the 1 between the hose and the vessel. keep the old 1 2 show the housing manigement after. good look pall