If your business or venue falls into one of the sectors or categories that should provide a customer log, and a visitor chooses to check-in using the official NHS QR code, they will not need to provide their contact details by any other route.
To confirm whether a customer has checked into your venue with the NHS COVID-19 app, you will need to ask them to show you their phone screen. They will need to click Settings then Venue History. The name of your venue should be listed in their venue check in history.
You will still need to offer a manual option for recording visitors' contact details, for people who do not have a smartphone or do not want to use the NHS COVID-19 app, or who do not wish to show you their venue check-in history.