OK so I found this, but the info below does not label anything.
From looking at the diagram, it would seem to me, the spring loaded valve on the end opposite the handle is the spring tension you feel when using the valve, and so the spring valve next to the handle must be the relief valve?
My next question is do I simply undo the nut that holds it together, to check and free up the relief valve? I fear if I turn it at all I may be adjusting it, and I have no pressure gauge to test with.