I cannot source Belgian dark beer for the carbonnade, will any dark beer do for this dish? I have a Yorkshire dark ale which is quite strong - will this substitute?
Nigella's Carbonnade A La Flamande (from KITCHEN) braises beef shin in a combination of beef stock and Belgian dark ale (which gives it the Flamande name). Belgian ales differ in style but generally are not as bitter as British dark ales. If you have a dark ale that it not too bitter then that should work in this dish.
We would avoid using stouts, such as Guinness, as they are too bitter for the carbonnade. However Nigella has a beef stew with Guinness in AT MY TABLE which adapts for the more bitter flavour of stout. Milk stout is sweeter and you may like to try Nigella's Oxtail With Milk Stout And Marjoram.