Answer - Thanks to Leander for the advice, the default Filmic colour space in Colour Management had to be set to Standard for the render to come out correctly.
One thing else to check, if you are setting up a Grease Pencil object from scratch and light is having an effect on the color: With the grease pencil object selected, inside Properties panel, go to the Object Data Properties tab. It's the tab that uses the green squiggle line.
In the tab, you should see your Layers expandable menu. Expand it and look below the Opacity slider, you'll see a checkbox called Use Lights. If you click this off, your grease pencil object will display colors as expected, if you leave it on then you can light the grease pencil objects using a normal light source.