Henderson_86 thank you for your reply. I've been using mail rules for many years (so am familiar with the support article you highlight) generally without problem. I have a lot of rules which I like to keep up-to-date and get rid of those which I no longer require. Before Big Sur 'removed' rules stayed removed but in Big Sur they seem to be deleted when I remove them but when reopening the app or rebooting the computer they always come back. It seems to me that the information to remove a rule is not being actioned/communicated by the system such that some plist or whatever is not retaining the command to delete as it should. This is a point which has been highlighted by many others over the years but I have never seen a satisfactory solution.
Regarding resizing the Preferences Rules window; In the past it has been possible to re-size a window by dragging the bottom right corner. This still works in many applications but for some reason not in this instance. Again, this is a problem that has been highlighted many times in the past but I have not yet found a satisfactory solution. It has worked before so you would think a company as bright as Apple would be able to fix this tiny glitch.
Just to be clear I have done all the usual PRAM, Safe Mode, Disk First Aid, Bend-Over- Backwards tests and manoeuvres that are usually suggested. It would be nice if some-one would find a solution.