Boiler Pressure Drop Before Cold Night

Your boiler keeps dropping pressure and cutting out, usually the night it turns cold. This walks the most likely causes in order, expansion vessel, weeping relief valve, radiator leaks, internal heat exchanger, tells you which you can check yourself and which needs an engineer. Diagnosis before you sit shivering.

My boiler keeps losing pressure. It drops to {CURRENT_PRESSURE_BAR} bar within {TIME_PERIOD} and then the heating cuts out. The boiler is a {BOILER_BRAND} and is showing error code {ANY_ERROR_CODE} or no error. I've already tried {WHAT_YOU_TRIED}. Give me a fault tree that covers the most likely causes in order: expansion vessel failure, pressure relief valve weeping, radiator leaks, internal heat exchanger leak. Tell me which checks I can safely do myself and at what point I must stop and call a Gas Safe engineer.