Thanks for taking time to read and respond to my query. This is a 64 bit machine around 2 years old and has only used Win 10. The BIOS is up to date using the Gigabyte motherboard built-in tool.
I am unfamiliar with a POST report other than the "beep" system from an earlier PC I had. This machine doesn't "beep" during startup, nor does it show a startup screen other than a welcome screen.
I don't have Memtest (and it looks a bit complicated for me) but I have run the Windows Memory Diagnostic Tool. It shows no problems in the reports.I suppose my next task is to remove one of the two RAM sticks as suggested and see the result.
I have copied below a minimised report from the "Speccy" program showing the basics of this machine.
Operating System
Windows 10 Home 64-bit
CPU
Intel Core i7 3820 @ 3.60GHz 41 °C
Sandy Bridge-E 32nm Technology
RAM
16.0 GB Single-Channel DDR3 @ 799MHz (11-11-11-28)
Motherboard
Gigabyte Technology Co. Ltd. X79-UD3 (SOCKET 0) 37 °C
Graphics
ASUS VH197 (1366x768@60Hz)
2047MB NVIDIA GeForce GT 710 (Gigabyte) 42 °C
Storage
232GB Samsung SSD 850 EVO 250GB (SATA (SSD)) 36 °C
1863GB Seagate ST2000DM 001-1CH164 SCSI Disk Device (SATA ) 38 °C
Optical Drives
HL-DT-ST DVDRAM GH24NSD1
Audio
High Definition Audio Device
Operating System
Windows 10 Home 64-bit
Computer type: Desktop
Installation Date: 8/08/2019 12:02:14 PM
RAM
Memory slots
Total memory slots 4
Used memory slots 2
Free memory slots 2
Memory
Type DDR3
Size 16384 MBytes
Channels # Single
DRAM Frequency 799.9 MHz
CAS# Latency (CL) 11 clocks
RAS# to CAS# Delay (tRCD) 11 clocks
RAS# Precharge (tRP) 11 clocks
Cycle Time (tRAS) 28 clocks
Command Rate (CR) 1T
Physical Memory
Memory Usage 33 %
Total Physical 7.92 GB
Available Physical 5.28 GB
Total Virtual 11 GB
Available Virtual 7.60 GB
Generated with Speccy v1.32.740
The Windows System still only shows 8 GB of RAM.
Thanks again for your interest and advice,
Copy that.