hi Stuart,
Yes a leak is a possibility, or when your heating system is hot, does it ever reach a pressure of above 3bar? if so the safety valve may be lifting and letting water out, this would be a sign that your expansion vessel needs charging or replacing.
Also, just check the safety valve is not dripping, sometimes after a lift (maybe someone disturbed it) dirt gets caught under the seat of the valve and it will contiue to drip forever until your system is drained and the valve cleaned or replaced.
T, April 2008