OK, so I did some more swapping and testing and I can confirm the following:
The "old" board has no "made in..." branding; but the new ones have "Made in China" silkscreened on to them.
I got two 32G C4 SanDisk SDHC cards, identical in every way to the SDHC card upon which development was done, wrote a virgin Raspberian system on one, and cloned my running system onto the other with win32diskimager.
I put the cloned SDHC into the "old" Pi, and booted it. No problems, everything worked fine.
I put the virgin Raspberian into the "new" Pi and booted it. No problems, booted into raspi-config. I didn't bother running that, but it did at least hint that there was no issue with card/socket necessitating a clothespeg.
I then got the "old" Pi, with the cloned SDHC and ran:
sudo apt-get update
sudo apt-get install rpi-update (it told me no need, rpi-update was already the newest version)
I could see update running, everything seemed fine, no errors, it completed and asked for a re-boot, which I did by power-cycling. After this, it came up and worked flawlessly.
Next, I took the apparently-good SDHC out of the "old" board (power off first
) and popped it into the new "Made in China " board. Wired everything up, powered up, still nada, zilch, zero, sweet FA
When it was first powered, the red power LED came up and stayed up. The Green OK LED flashed briefly after power up but not for long and only once. No other LEDs came up, and the screen (HDMI connected) remained blank.
I didn't have a clothes-peg to hand, but I used my standard-issue thumb and forefinger to compress the SDHC into the socket as described but it made no difference. I hadn't really expected it to because the virgin Raspberian SDHC worked fine in it, suggesting its a problem with the boot sequence, not the physical connectivity.
I'm a bit worried I will have to manually transfer files from the development machine onto the others if I can't reliably clone SDHC cards, but I find it hard to believe that is necessary, and I'm suspicious that I've missed a trick somewhere along the line.
Oh, and just to stop the obvious question about the compatibility of the SDHC cards, I have cloned onto two different cards and got the same failure with both, and blown virgin Raspberian on to the SAME two SDHCs and got faultless boots to raspbi-config
If anyone has any further ideas, I'd be real glad to hear....