iphone 16 not backing up despite sufficient icloud storage

My iphone 16 has stopped backing up to icloud. I have a 50GB plan and have just short of 20GB free. It used to back up and the backup size was c 4.7GB so still had around 15GB unused until about a week ago when I got a message that the backup failed. Each time I try I get directed to a page to buy the 200GB version of icloud. I have tried several of the online suggestions including deleteing the last backup, after which i have 20GB free but the backup still fails and now i have lost even the old version of the backup. iOS is up to date and all the normal checks are fine, so I am a bit stuck as to what else to try. I certainly don't want to upgrade to the 200Gb icould storage option as i can't see anay reason that I should need to. Any suggestions?