Halsted Finest Gold going over pressure ??

Hi everyone,
My Halsted Finest Gold combi boiler works fine but i have to top the pressure up every day. I set it at 1 bar but when the system heats up, the pressure soon rises to 3 bar at which point it vents itself to lose this too high pressure. Obviously when it cools down overnight, the pressure is nearly zero and boiler trips out.
I switch it off, boost the pressure back to 1 bar, switch it back on and it works again. I need to know what is causing the pressure to rise like this, as my plumber just said " you need a boiler mate" yeah right, sooner fix it myself.
I am an engineer and have no problems replacing parts or even repairing them, but first i need to know where to look and wondered if anyone in here was familular with this fault and could point me in the right direction.
Thanks, Bob.

Problem is (probably) one of the following:-

(1) Connection to the expansion vessel blocked
(2) Expansion vessel has lost its charge
(3) Expansion vessel diaphragm is ruptured
(4) Plate heat exchanger is perforated

Well thats plenty of options, where to start though, would seem ive got to look at expansion vessel first then
Thanks

Empty the system pressure to zero and test the exp vessel pressure. If water comes out it is completely knackered. Pressure should be around 10 - 16 psi.

If it isn't re-pressurise by searching faq's on here.

if the vessels shot, to change an ex vessel on these, the boiler has to be removed as it stays on the mounting jig. Save your self the hassle and fit a remote one.

What kind of prices do the external pressure vessels come in at, i googled them and finished up everywhere except central heating places lol,
Bob.

Fifty quid(ish) including wall plate.