Thankyou for your help Dave. I followed the video in the link Laura Stone link you sent and although it didn’t seem to answer my problem – I didn’t have any presets – it got me thinking about monitor profiles. I went on the Lightroom Queen site and found the answer to someone’s query about Library and Develope not matching. On checking the Colour Mangement control panel, I changed the profile from the Dell default to sRGB IE61966-2.1 . And voila – everything back to normal!
So thanks again Dave – I can sleep easy tonight.