iOS14 Health - Mobility

Can anyone help explain why I can’t get my iPhone6S running iOS14 to collect and show any data in the Mobility section of the Health app? My number of steps and flights of stairs are appearing in the Activity section as they have always done but the equivalents including Step Length and Stair Speed Up or Down in Mobility are not. I have tried every menu to see if I need to switch on something but not found anything!

Clive mentioned to me separately that he’s also wearing an Apple Watch Series 4, and I can’t think of any reason both the iPhone and the Apple Watch would be missing that information.