Experts are concerned that Alan Ayckbourn’s plays may soon face extinction. Fewer than 80 of these precious beasts still exist in their natural habitat, so theatre-goers will be cheered to know that the National Theatre has created a genetically identical replica and released it into the wild. Rules for Living, fashioned by Sam Holcroft from the Acykbourn blueprint, is a bourgeois natter-fest in which bickering couples meet for a fractious Christmas summit.