Hmm - the issue is that I'm trying to run @j0ju's replacement firmware from SD card, and the boot process appears to be dropping-back to TFTP/pxeboot because the image is unable to select a DTB for my board. Were there different releases of 2.4 boards - @j0ju's apparently reports

tpi_hw_version=v2.4.0

and their device-tree model is

Turing Pi 2 (v2.4)

once booted. (I've previously followed @svenrademakers instuctions to set a static MAC address in flash – it's possible I got this wrong somehow!)