4 month old Vaillant ecotec plus 831 falling pressure?

new boiler worked for 6 weeks after installation in Oct 2008 and then has consistently suffered from falling pressure-discovered installer had installed extra expansion vessel on previous boiler but they refused to remove from new one . Asked experienced trusty plumber(has done many jobs for me) who disconnected it and it settled for 6 days and then lost pressure again-F22.Any ideas? getting desperate! Diane