There are a couple of distinctive differences Stuart. Firstly the pattern on the apices is different. Amplipennis has a dark mark right on the elytral tip whereas subpura has a small mark on the margin just above the apices. Subpura has a yellowy/light brown background whereas amplipennis has a reddish brown background colour (a bit more like rufipennis). Amplipennis is also a larger species than subpura.
Thanks Mark, in retrospect I will go look at my sightings because I may have confused these 2 x spp. Colour variability can be tricky so the apical makings will be useful to differentiate..