Many thanks for the detailed feedback.
I can confirm that I do NOT have any Sangoma hardware. I was pointed to the wanrouter start / stop level mod by @tm1000, which is why I tried that.
I accept that the hardware I am using is NOT digium manufactured, but there is another member (@glew) who is reporting exactly the same symptoms as me, and he has indicated that he is using genuine digium hardware.
I’m not sure if he’s tried his system with the ‘broken’ wanpipe driver. I saw him posting today asking how to go about the big update that came out the other day. However, I guess that if he does the update now, he’ll miss out on the broken wanpipe driver, so he still might not get DAHDi to work (unless he then downgrades wanpipe to the broken version, as I have done).