Let the system cool and repressure it. Check the PRV outlet. If water is coming out at 1.5 bar then the PRV is probably faulty.

If not then turn it on and watch the pressure guage. If the pressure climbs upto 3 bar and _then_ water comes out of the PRV it's probably the expansion vessel. The expansion vessels job is to allow the water to expand as it's heated. Without an expansion vessel (or with a faulty one) the pressure can increase until the PRV opens. The expansion vessel might only need represurising or it might need replacing. If the expansion vessel is burried deep in the boiler it might be cheaper to add another one on the outside of the boiler rather than replace the one inside. You can leave the faulty one in place if it isn't actually leaking water.

It could also be both the expansion vessel and the PRV. Once a faulty expansion vessel starts causing a PRV to open the PRV tends to be susceptable to leaking.