I have bought a Hach HQ40D for measuring boiler water and the supplied gel filled electrode is giving some erratic readings, have I got a faulty electrode?

The gel filled electrode is a general purpose electrode designed for a variety of applications, the measurement of boiler water is a specialised application and requires an electrode designed specifically for the job of measuring Low Ionic Strength water, such as boiler water. There is an electrode available for the HQ meters for Low Ionic Strength solutions here.