I found that neither flashing the enhanced kernel, nor flashing back to the stock Nokia kernel made any difference to promisc mode.

However, I have now re-flashed from the USB cable and this time, I was pressing the U key as I connect the USB cable as per the wiki instructions. Pleased to report that this time promisc mode has been restored! As a side note, I think I failed to press the U key during the last couple of re-flashes. Does that really make a difference? Perhaps it does!

I would conclude that it is not a kernel issue, but more likely something to do with a driver for the wireless chipset that gets replaced without warning during some package upgrade. I hope to work out the offending package or at least work out a way of taking a backup of this precious config that can be restored without a re-flash. Once again, any help from people who actually understand this stuff would be most welcome!