Thank you for your help. Unfortunately none of the above helps. With the latest AirPods (USB C version) the AirPods graphic does not appear on an iPhone 8 (plus) when you open the lid of the box and the battery widget does not show the AirPods graphic. Also, the graphic is not present in the AirPods section of the bluetooth connections (under settings). Apple have removed the compatibility of the battery status with older iPhones from the latest AirPods - the previous version of AirPods (with a lightning connector) do work as you described.
If anyone with an old iPhone wants a pair of AirPods I suggest you get them from a third party vendor (e.g. Amazon) before they sell out (alternatively you could spend the best part of £1000 on a new phone)