Thanks for the comments.
We canot upgrade from V0.28 on Ubuntu 14.04 as we have tried. We are aware of this problem but the documents on moving to Ubuntu 18.04 (Mail-in-a-Box version v0.40 and moving to Ubuntu 18.04) explicitly state that this should not be a problem unles you are using Nextcloud, which we aren’t. It would be nice if MIAB had an option to not install it but we are where we are.
Not sure why not doing the backup restore test is scary. We know we have to do it, we just haven’t done it yet. We will be doing the restoration to test the system, I have no doubt we’ll do it a number of times.
We weren’t aware that the restoration is the issue for nextcloud. This is why we test.
I hadn’t seen the document [Solved] Workaround for upgrading over multiple Versions of MiaB to the latest when multiple major Versions of Nexcloud are included (first accured in upgrade from v0.30/v0.40 (NC13) to v0.42b (NC15) with v0.41 (NC14) between ) ! UPDATED! cose of backup. We will read it and learn.
We will look at setting a backup MX server. We will probably do that on premises rather than with our provider.
MIAB is not handling any DNS at all. We use easyDNS in Canada. It’s all managed from them.
We have five domains and 20-30 users per domain. Its not big, but its not small.
The move from one server to another may be really quick, but we want to make sure its really quick by doing it a number of times and testing it so we find the issues in office time, rather than at 03:00 in the morning after nine hours of problems 
Thanks
rob