This can happen at any time but usually when battery is showing less than 55% charge. It just suddenly drops to 0% and shuts down.
When put on charge and turned back on it immediately shows battery level at whatever it was before shutting down.
All current updates have been downloaded.
Any sugestions please.
How Long have you been using the phone? If it's two years old or older, you may be experiencing battery failure creep, and need a replacement. Had this happen to me years ago, and a new battery fixed the issue.