Pressure loss from MegaFlow sealed system

[pendennis]

Hi all, newbie here:
In the last few days I have been finding my CH not working due to no water pressure. In the past I have had to use the filling pipe once in a very long while but now it's down to zero in a day. No leaks obvious anywhere. System was installed in 2008. Yesterday I re-filled (closing the input valve at a mid-scale reading) and after some minutes of operation the pressure was off the UPPER end of the scale, leading to an error code on the display (although curiously the radiators did heat up).
I hope this is not duplicating a thread elsewhere. Can anyone help a non-expert like me work out what is happening? Throughout this I've had hot water in my shower ok.

 

[Dan Robinson]

Dead expansion vessel.

 

[pendennis]

Thanks, Dan. The brevity of your reply makes you look pretty sure of this... How expensive is this to get fixed? Something that a sensible amateur could do? (I'm quite good at carpentry and most DIY, no plumbing experience though)