Heating system?

Hi, I have a eurocombi SX 20 MFFCE & when my central heating is on & then switched off I lose pressure from the system, the water is going out of the exhaust, I then have to top it up, to put it back to 1 & half bar, this problem has been steadily getting worse, I now can only run my heating for approx 1 hour.
my hot water is working fine, the diverter valve was changed last year. I have been told by a plumber that the heat exchanger has gone.
I have looked in the manual & its saying an expansion vessel, is this the item to change?, it has a bleed cap which can be unscrewed. It looks an easy item to replace so I will do this myself, after draining the system of course.
All advice would be apprecieted
Kind Regards to all