Thanks for the pointer about formatting the data.
This morning I managed to get the pool imported read-only using some zfs suggestions at https://github.com/openzfs/zfs/issues/13483
specifically on the linux command line I added zfs.zfs_recover=1 zfs.zil_replay_disable=1 and ran the zpool import -f -F -X pool1 command. That glagged up erros but imported the pool as far as I can tell.
That allowed me to get the UI up and take a backup of the configuration.
This didn’t fix the panic on the next boot so I’ve decided to re-install and as you say see if a fresh install works. If not I’ll nuke the entire box and start afresh, I don’t have much spare time to start digging further as this box also provides shares for the rest of the family and my business.
Thanks
Pete