Many thanks - everything worked perfectly.
The conversion appears impossible to do without first creating a contiguous file at 12.5% the size of the drive to be converted.
The command is: fsutil file createnew x:\MFTFile.txt 3750000000
where x is the drive to be converted and the last figure representing 12.5% of the drive - in my case this is a 30GB drive.
The created textfile may be of any valid name. Once the file is created, it is vital to run a defragmenter to ensure the file is wholly contiguous.
Then use the command: convert x: /fs:ntfs /cvtarea:MFTFile.txt to convert the volume. In the case of a non-system volume, conversion takes place immediately. A system volume will offer a choice to unmount the drive - select NO. Then will follow an offer to convert on the next bootup - select Yes

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An interesting feature is the use of the command cvtarea. Apparently this command forces the MFT file to be contiguous as would be the case if the NTFS system had been formatted when the drive was first installed. Otherwise the conversion may not be as perfect as one created during setup.