Vokera Linea 726 - Pressure relief valve vs re-pressurise?

Hi. I have a Vokera Linea 726 which drips constantly to the outside but much moreso when the central heating is on (so much that it is like a steady stream). Checking the pressure in the system while the CH is on reports about 2.5 bar. The pressure relief is set to lift at 3bar (instructions say).
I have read previous questions/answers and think that problem could be either the pressure relief valve or the expansion vessel. I am leaning towards the relief valve due to the constant dripping. (This even occurs when system is set to 1bar).
Does anyone have any thoughts on this and should it involve a professional callout? Thanks for any advice.

Martyn Thompson
March 2007