What should you do if the pilot won't stay lit on a Teledyne Laars pool heater but will stay on as long as the button on the control unit is pushed down but the flame goes out when released?

This is probably due to a faulty thermocouple. The thermocouple generates a small voltage when hot that is used as a security device that keeps the gas flowing to the pilot. These are relatively inexpensive (UK about �18).

You can check if this is the problem by measuring the voltage on the wires coming back from the pilot. When you have the red switch depressed, check the voltage (should be about 30mV). If there's no voltage, then change the thermocouple.

The thermocouple will generate up to 50mV in a pilot flame. If it falls below 20mV it should kill the fuel source. It may take a few minutes to generate enough voltage in order to keep the pilot lit, so you may have to depress the button for a minute or more. This would be typical if your unit has been off for an extended period of time.