Look carefully at the pressure gauge, if it jumps about when heating is on and goes from 1 to 3 or so quickly, then it is the expansion vessel. Otherwise I agree with the other post that it is the PRV. goood luck.
Colin, Chris, Chunky, Kev, Ozzie, Ron, Whizzer, me Muller,Pete, Terry, Brian ( Use me ) Dildo, many more but this was the core of the team, are you any of these or can you name those I have forgotten.