AppleID password changed

I recently changed my AppleID password using my iMac. I was able to login OK with the new password on my iPad and iPhone, but the Watch keeps asking for the password. Although there is a blank square to enter it I cannot see any sort of keyboard? There is a Key symbol which when tapped brings up a list of email addresses and websites from my Keychain, but when I tap the relevant one for AppleID it gives an invalid password or username error.

I have tried restarting the Watch, but no change. I also tried updating the software, currently 8.7, but was told I need to update the software on the iPhone, this is up to date 15.7 (altho' 16.0 is available)

Any ideas how I can resolve this?