vaillant vcw-sine18t3fdpb

Boiler is mounted high level {4 inch of celing).
Signs of expansion vessal failure are apparent ie 2.7 bar at 60 degrees/ 1 bar when cold.
Signs of leaking are apparent on underside of heating vessal (on right) and drips from underneath boiler when hot.
How can i gain access to the shraeder valve which i presume is on top of the vessal to inspect whether this is source of leak?
Is this boiler off the wall job to inspect or change?
I have no boiler instructions and can not find any!
Is there any thing stopping me from installing a new vessal close to boiler on the return and do you think this will stop the leak as i presume it is pressure not temperature causing the leak.
i dont want to go to out of my depth as this combi looks as complicated as they come and i have no instructions and im an installer not a break down engineer!
the boiler operates correctly in all modes!
is this a common fault the shraeder vavle leaking meaning the diapham inside split ?
should the valve still not be water tight untill depressed?
if my diagnosis is correct whats the best course of action?
if im way out on my diagnosis can u point me in the right direction ?
the customer is moving out and new boiler is kind of out the question!

thanks guys

in need of knowledge engineer