Thanks for the above comments/suggestions.
This in the .cfg file works to allow reading 'sector 0' :-
mount c c:\users\roopy\source
mount -t floppy b: c:\Users\roopy\Downloads\BackupDownloads\iso\DOS5and6
imgmount a -t floppy Dos5.img
It means the code I'm testing can be run as a DOS command (temporarily) and it reads the boot sector from the ...
DOSBox only allows absolute sector reads from disk images, as mounted by IMGMOUNT.
You can’t use interrupt 0x13, service 0x02 on drives mounted with MOUNT. (To understand that link, note that imageDiskList in DOSBox is only populated by the BOOT and IMGMOUNT commands, not by MOUNT.)
The workaround is to use IMGMOUNT:
imgmount -t floppy a /path/to/floppy.img