Just been through a similar thought process after doing a Mode C check with Radar. They had me 200' below my indicated Altimeter reading set to the QNH they gave me.
On the ground when I set the airfield QFE I am at -100', for an elevation of +25'.
Had a look at the altimeter but no way to adjust externally, so probably have to replace it or send it somewhere to be calibrated. Any ideas?