It sounds like a Windows problem rather than Photoshop. To make Photoshop the default program for whatever sort of files, navigate directly to the Photoshop .exe file in your C: drive.
Being continually asked if Photoshop can make changes is something else. First thing to try is resetting Preferences
I'm thinking this this should fix it, but if not, then I wondering if you have a permissions problem.
Navigate to this folder, and select all (Ctrl A) Right click and if anything is set to Read Only, un check it.
C:\Users\Trevo\AppData\Roaming\Adobe\Adobe Photoshop CC 2018\Adobe Photoshop CC 2018 Settings
Thansk for this. I suspect it may be a Windows issue as, when I select a file to associate with photoshop, PS is not listed in the "available apps" and the only option I can see is "select an app from the store" - I do not get the option to select an app on the PC which is what I have used on previous occasions.
None of the files were read only and resetting preferences did not help.
You have to scroll way down to see the 'Look for another app on this PC'. Windows would have to be very broken for that not to be there.
You can also use Bridge Preferences, but this would only affect file opened from Bridge. Plus you can override some of those file associations by using Ctrl r to open files like JPGs in ACR.
It might be worth using sfc /scannow to look for and fix Windows issues with corrupt OS files. It doesn't fix everything though. Not by a long way. But it is worth a try.
Thanks for further help. I have managed to sort the issue by using Windows File Explorer, selecting a jpeg file and right clicking to "open with". This allowed me to select PS CC 2018 and also to "always use this app.."
The newest WIndows 10 release seems to be fullof "tricks" to drive you down the MS app route with what appears to be an extra click here and there to do what you want to do!