Aha! I seem to have cracked it!
Whilst looking for “Preferences > Image Processing > Default Processing Profiles”, (which incidentally I never found) I noticed, top-right, “Processing Profiles” under which it said “Custom”. I changed it to “Neutral” and the dark cast cleared. Sorted!! In my fumbling inexperience I must have adjusted the Processing Profile on the image and it appears that the software will, by default remember the “Last Saved” profile when closing.
Thank you all for your assistance. I have a strong suspicion that I will be back soon for more help.