Problem Solved!
When Rendi asked me the question regarding which o/s I am using, a light went on and I proceeded to investigate and ticked all the Permissions for 'Sharing' and Security' for the Prepar3D and and Prepar3D folders/sub-folders. And, for good measure, I created an additional category under 'Sharing' called 'Everyone' - tip from the OpusFSX OpusFSX_Getting_Started_Single_PC.pdf - thanks!
I then deleted everything from the 'Modules' folders in both sims except for the uipchist.bin and SimConnectP3D(2).dll files as I thought they might have had something to do with the original LM installations.
The FPUIC installer was then run once more as Administrator and an 'FSUIPC4 Install.log' file was again created in the Prepar3D' Modules' folder (but not the Prepar3D v2 'Modules' folder). 2 other files were missing from the Prepar3D v2 Modules folder: 'FSUIPC4.log' and 'FSUIPC4.key'.
The 'FSUIPC4 Install.log' was examined carefully from top to bottom and all appeared well including the following text at the foot of the file:
"All installer tasks completed.
Registration for FSUIPC4 was successful! (result code 00)"
So, feeling confident, I ran both Simulators again and no issues appeared this time.... except FSUIPC was still unregistered in Prepar3D v2! Bummer!
Then, I had a brainwave: I copied the 'FSUIPC4.key' file across from the Prepar3D 'Modules' folder and it worked!! All is now well, and both installations appear as Registered with all FSUIPC options available.
So, it would appear that the FSUIPC installer failed to copy across 3 files into the Prepar3d v2\Modules folder: FSUIPC4.log, FSUIPC4 Install.log and the all-important FSUIPC4.key file.
Apologies for the somewhat lengthy explanation, but I thought it might be helpful to those of you who still have Prepar3D 1.4 and FSX installations on your drives. Also, the Permissions side of things (turns out may not have been relevant to this issue, but is now being regarded by me as preparing the way for the future) was an education as, to date, I haven't encountered any related problems despite not having installed each simulator to C: root. I just accepted the default paths for installation.
Mike