Thanks to the forum members here for pointing me in the right direction about this (I believe it was Dan) but XML import can place explicit items where one might not notice them and this can lead to certain problems with layout. My original problem was about the inability to hide staves which were set to hide, and it turned out that the XML process had placed clefs on each staff at the beginning, which Dorico interpreted as essential information, overriding the hiding of the staves.
Another problem I had was that the first bar of a section which introduced a new key would never be included in a multi bar rest. The problem with the clefs led me to check the key signature and sure enough, clicking on one of these would highlight only the KS on that staff or on a small group of staves. Deleting these didn’t change the overall KS but it did enable the first bar to be incorporated into a MB rest. I check the key changes of XML imports by simply selecting the KS. If all is well, the KS in all voices should select all at once (including the flag if there is one).
I hope this helps anyone else who’s had these problems with XML import.