Alpha CX28 expansion vessel?

my son has bought a house with an Alpha CX28 boiler 16 months ago I replaced the expansion vessel as it was throwing out the water regularly. In the last two weeks it has began doing this again. Any ideas and if it is the same problem this time I intend to fit it adjacent to the boiler on the wall. ( replaced the original by removing the boiler from the wall etc) any ideas on the expansion vessel type (wall mounting) and pipework information please.