Unlock iMac with Apple Watch

The facility to unlock the iMac with Apple Watch has worked continuously since I first acquired my first and subsequent watches. I have obviously been to preferences to "Allow Apple Watch to unlock this iMac". No idea why this switched off however I attempted to switch it on and received the following message:

Your Apple Watch must be signed in to iCloud to allow it to unlock your Mac.

and the following message:

You can sign in using the Apple Watch app on your iPhone.

Checked that out and unfortunately the watch is already signed in to iCloud (with the same Apple ID) so unsure what to do next.

Note: All WIFI, Bluetooth etc. fully enabled and both devices are utilising the same network. So what to do next? is this a "switch off and switch on" scenario.....