In order to determine if the leak is within the boiler or not, close the isolating valves on the flow and return pipes underneath the boiler. You won't be able to use the heating while they're shut.
After say 12 hrs note whether any pressure drop is shown on pressure gauge. If so, the leak's from the boiler. If not, see what happens when you open the isolating valves. If the pressure then drops the leak is in the pipework. If no drop occurs, the leak only manifests when the heating is on!
After say 12 hrs note whether any pressure drop is shown on pressure gauge. If so, the leak's from the boiler. If not, see what happens when you open the isolating valves. If the pressure then drops the leak is in the pipework. If no drop occurs, the leak only manifests when the heating is on!