John Woods wrote to the BBKA to share some information about entrance 'muzzles' that he uses to help the bees against Asian hornets.
He has collated information about 'muzzles' to help protect bees from Asian hornets. He suggests not using a small mesh, but the 'lyre' type he's created seem to work best.
He has collated this information with help from other local beekeepers


I live in central France and had bees occupy an old bird winter roosting box on the end of the garage, they were there for seven years minding their own business. One year Asian Hornets arrived and I didn't notice as they were a 'wild' hive so I left them alone.  By the time I noticed three hornets were regularly there, they pinned the bees down and being a small colony (35L box, what were they thinking ?) they couldn't stand the pressure and died out.  Larger colonies can take more punishment but with all their other problems this is one they can do without. 
I spent a year without bees and now have two new hives, my first year of actual intentional beekeeping, and wanted to make sure hornets were not going to do this again.  I checked around and researched and collated the information on how to protect the hive itself in one place. Many people have provided ideas and information and thanks to them for that.
France has lived with this a long time and now Spain, Italy and others including the UK are starting to see these.
The muzzles are a simple thing to give the bees breathing space so worth doing.  Main criteria for such protection is to let the bees come and go freely at full speed, and irritate the hornets so they look for something easier. I previously had two hornets every hour, currently as it is colder less than that, which may just be good luck or may be they have found easier pickings elsewhere. 


Link to a video showing the lyre muzzle in use