Upon some sleep, I'm guessing the issue is that the instructions I followed either assume you aren't removing the primary PV or don't care.

I guess the lvm metadata is stored on there, which contains VG and LV information, and upon doing 'vgextend system /dev/newpv' it doesn't mirror that information to the new pv.

So by removing the old pv, I've shot myself in the foot.

I guess I should have used vgcfgbackup/vgcfgrestore (which I've now discovered).

Any other light to be shone on this situation?

The good news is that this is just my os/home drives, and I have a semi-recent backup of the /home part, so I'm not that worried about reinstall and restore.  Clearly I'd prefer not to, but it wouldn't be the end of the world.

cheers,

      / Brett