Sorted it - hurrah! For those who may be as stupid as me, all you need to do when on secondary devices is to create a new vault. I called it exactly the same name (just in case). Then I turned on sync. This asked me to sign in. Then I could see my available vaults to sync with. Selected the vault of the same name and away I went. I made sure NOT to select the option Sync with iCloud. All currently synching (hopfully) fine.