Vokera linea 24 pressure problem?

I have a problem with my Vokera boiler, when cold the pressure will drop to almost zero, if I top up the pressure to 1 bar and turn on the heating it increases slowly to about 2.5 - 3 bar(at 3bar the PRV kicks in ok)
Once the heating reaches the right temperature (70deg) the burner stops and the pressure starts to fall, once the temp reaches around 58deg the burner starts and the pressure increases again. If I switch off the heating it slowly drops pressure to almost zero over about 30 mins. I found there was no pressure reading from the expansion vessel so I recharged it to 15 psi but problem still persists.

Can anyone help.