Ok, so quick update:
I stripped everything back and just animated a leg with no IK bones, or targets or anything. I’ve now got it correctly importing into UE4. Key to getting it to work was not touching the weight painting. Everything seemed to go hell in a hand basket if I touched the weighting (UE4 crashed when I tried to import it).
I’ll now try and extend my armature to a more sophisticated model while being careful to maintain the same settings.