Thank you for your reply, Paul.

I'd already tried the Repair option before the previous posting, without success.

I carefully did the procedure outlined. Unfortunately, after about 20 corrections on the new file, the problem resurfaced, as before. Equally unfortunately, the new file had lost all the Markup corrections, assuming they were all correct without my editor reviewing them.

I may be talking nonsense, but my impression is that Word is restricting the availability of memory, disk space or some other feature during editing of the document. A system summary, with Word running, is below:

OS Name Microsoft Windows 10 Pro
Version 10.0.14393 Build 14393
Other OS Description Not Available
OS Manufacturer Microsoft Corporation
System Name BNE-MAIN
System Manufacturer Hewlett-Packard
System Model HP Pro3500 Series
System Type x64-based PC
System SKU D5R79EA#ABU
Processor Intel(R) Core(TM) i3-3240 CPU @ 3.40GHz, 3400 Mhz, 2 Core(s), 4 Logical Processor(s)
BIOS Version/Date AMI 8.17, 25/10/2013
SMBIOS Version 2.7
Embedded Controller Version 255.255
BIOS Mode Legacy
BaseBoard Manufacturer Foxconn
BaseBoard Model Not Available
BaseBoard Name Base Board
Platform Role Desktop
Secure Boot State Unsupported
PCR7 Configuration Binding Not Possible
Windows Directory C:\WINDOWS
System Directory C:\WINDOWS\system32
Boot Device \Device\HarddiskVolume1
Locale United Kingdom
Hardware Abstraction Layer Version = "10.0.14393.206"
Username BNE-main\Brian
Time Zone GTB Standard Time
Installed Physical Memory (RAM) 4.00 GB
Total Physical Memory 3.89 GB
Available Physical Memory 1.90 GB
Total Virtual Memory 4.14 GB
Available Virtual Memory 1.72 GB
Page File Space 256 MB
Page File C:\pagefile.sys
Hyper-V - VM Monitor Mode Extensions Yes
Hyper-V - Second Level Address Translation Extensions Yes
Hyper-V - Virtualisation Enabled in Firmware No
Hyper-V - Data Execution Protection Yes

Brian