Works Perfectly, thank you @pgr .
So all that I did was create the batch file in Windows Notepad. Called it Schedule.bat.
Be Aware- the instructions at the bottom of the SuiteCRM Scheduler screen should not be taken as is. The cd/D statement is fine (use your own path if different) but the second line should read something like <your path to php.exe eg c:\wamp64\bin\php.…>php.exe -f cron.php.
Otherwise the batch file will error and if you haven’t got a pause statement in there it will just flash through and you wont know unless you start diving into logs.
Once it runs, the SuiteCRM Scheduler works like a charm and does what it says.
We have set this up to run in Windows Task Scheduler to fire up the batch file on system start-up so that it is there and running. This changed all of the records (840 in total) in background in about 5 minutes, with no failures.
We have now de-activated the old user and all works fine. We will do this on the production server this afternoon, so that everyone is happy come Tuesday morning 
This may seem like overkill but our overall objective is simple. rather than have to rely on skilled coders to do admin tasks, we want a SysAdmin to look after all of these tasks and processes from within the system. That way the developers can get on with key project activity and the administration of the system goes straight to the Sys Admin team who look after all application admin and security.
Many thanks for the pointer and detail on this which has also enhanced our knowledge of the scheduler, which may come in handy elsewhere.
Thanks again.