I am pleased to say that I have mananged to fix the issue.

As indicated due to the same error repeating as a last resort I uninstalled and reinstalled both programs and see if that helps to get things working correctly.

It took approx 2 days to download the complete blockchain, and fortunately there were no errors and both Bitcoin Core and Armory are working well.

goatpig - Thanks for confirming that running Armory version is 0.96.5-beta-8730390989 together with the full 648 Gb blockchain of Bitcoin Core 27.0 (64 bit) is capable of working on Win 11.

nc50lc - Thanks the tip regarding ghost processes which I will use next time if I encounter a similar problem.

From this excercise I have learned the following:

 1. If all else fails re-download the entire blockchain as it takes less than 2 days (and not just re-index it).

 2. In my specific case the Armory program did not behave as per the user manual. Entering the directiries for Bitcoin Core, the blockchain and Armory databases (for my setup locations on a spare fast SSD), into FILE / SETTINGS made no change to the program. Neither did placing the same information into the armoryqt.conf file. The only way I could get Armory online was to use suffixes in the desktop shortcut i.e. using
[ Z:\Armory\ArmoryQt.exe --satoshi-datadir="Z:\Bitcoin Core Data\blocks"  --dbdir="Z:\Armory Data" ].

Hope this helps anyone finding themselves in the same situation as myself.