1. In a fluid delivery apparatus comprising: a flow line for delivery of a fluid through said flow line, the flow line comprising an upstream, section downstream, and an section intermediate section between said upstream and downstream sections: a closable upstream valve and a closable downstream valve, said valves being interconnected in series in the flow line through said intermediate section, a method of detecting a failure of a said valve to close fully wherein said method comprises the steps of: (a) putting both said valves in a fully-closed setting; (b) detecting the pressure of said fluid in the intermediate section; and, (c) initiating an alarm condition, but only if the said pressure fails to change in a predetermined way, wherein step (a) comprises putting the downstream valve in said fully-closed setting before putting the upstream valve in said fully-closed setting, whereby the alarm condition is responsive to failure of at least one of the said valves to close fully.