For some reason I can no longer edit bitmaps in either 2018 or 2019. It seems to have occured after I uninstalled the trial version of Coreldraw 2020 Standard I had taken down to see if it was worth getting for a young relative (It wasn't).
It's OK now. I repaired both programs and got the edit bitmap facility back. It was obviously uninstalling the CD Standard that caused the problem but no idea why.