expansion vessel?

Hi have just had a external expansion vessel fitted to my Lineg 24 after losing pressure and advised to do so by plumer. Who was on site for 40 mins and charged me £180 cash. The problem is I am still losing pressure
at no time have I seen any water come out of the overflow. So were does the water go??? any ideas what the prolem is question should the old vessel be isolated and how can this be done?? have been under floor to check pipe work and found no leaks.
Regards Len