More info: I force-flashed the router back down to (lede) release 17.01.4 and I see very similar errors on boot, so I now think that my previous "belief" (that I was "running lede 17.01.4 with no errors") is incorrect. I now think that the nand on this router is failing and that therefore the router needs to be binned/thrown away. I have attached the "17.01.4.bootlog" - could anyone please confirm whether or not the nand on this router is broken? Many thanks.
OK, more info. Instead of reflashing by running sysupgrade from within a running openwrt, I booted the initramfs from the Porta-Danube prompt and then (once that had booted) I ran sysupgrade from there. This worked perfectly for the first couple of hours (no more nand/ubifs errors at all) but then the router rebooted itself (?!), and then the nand/ubifs errors all returned. I'm beginning to think that the nand/ubifs errors are simply the result of instant reboots caused by either me pulling the power or kernel crashes combined with a new sync semantic whereby an unclean shutdown will trash the ubifs filesystem (I'm guessing here!!)
I'm going to bail out of this and go back to 17.01.4 until I have the time to work out what's going on.