Thanks again for that loughor. Before I install I'll obviously do a TM backup and a CCC 5 bootable clone of Sierra, talking of which I was reading Mike Bombich's blog to see what was happening at CCC headquarters. To say it was illuminating is an understatement. The man has certainly checked his product out to make sure it works with HS. Fact and figure kept tumbling out. Read his blog here
https://bombich.com/blog very reassuring and interesting. Thanks Mike and his team.