So the final step on my odyssey (possibly)
3D printed case (link above), a right angle header and a short cable made up with a crimp tool makes for a very clean setup. Pi is then powered off the 5V DC to the emonTX (it works the other way as well).
End cost is slightly more than the emonESP, but far more flexible, can update firmware and no need for emoncms on a second machine/Pi. My aim is to install this at a friend’s house for a month and see if Agile will save him money. If it does the £50 referral will more than cover the cost of the Pi & case!