A brief update on progress (or lack of it).
I decided to experiment with the settings in msconfig after first making a backup of my C drive. The maximum memory showing in the advanced boot tab was 12gig, which seemed more than a coincidence as this was the maximum I was able to install.
Turned out it was just a coincidence. I'm a little out of my depth here, but it seems that msconfig was monitoring the memory available rather than providing an opportunity to increase some arbritary limit.
I increased the DRAM voltage in small increments but was still unable to get past the Windows boot GUI with 4 sticks (16Gig) of memory installed.
I adjusted the memory speed down to the next lowest setting with and without a reduction in voltage, but still no luck.
I spent several hours searching the 'net, for anything relavent but found nothing.
I also posted an email question to Gigabyte which has been acknowledged, but I don't hold out much hope of a useful reply.
I think it is most likely that there is a compatibility issue with the memory. Sometime in the future I may try a matched set of four memory sticks. For the moment I have carefully repackaged the two upgrade sticks and returned them for a refund.
Now the kettle's on and I intend to sit down with a nice mug of coffee ... who needs an upgrade to 16Gig anyway ??
Thanks to you all for your tips and advice.