Many thanks, exile 360, for your advice.
I carried out your suggested actions viz:
'Open Malwarebytes and under Settings>Application find the "Windows Action Center" setting and select the option "Never register Malwarebytes in the Windows Action Center" then restart your PC to see if AVG returns to normal with Malwarebytes' protection active'.
On restarting my computer, I found both AVG and Malwarebytes are happily co-existing together again:
AVG reports: 'you are fully protected' with the usual 5 green ticks.
Malwarebytes reports: 'awesome - you're protected' with 7 green ticks on their dashboard.
So, all good now. Thanks again for helping me sort this one out.
ackagee