It doesn’t take a genius to work out that there is an easier way for a law firm to operate internationally than by going through a complex merger.

Allen & Overy’s protracted talks with O’Melveny & Myers are a good example of how long it can take to agree to the details of a large-scale tie-up. That is, if it happens at all. And things are rarely straightforward afterward either. One year on from its trans-Atlantic merger, Bryan Cave Leighton Paisner’s management believes its 1 percent rise in revenue and 5 percent rise in profits were “great” results given the “distractions” involved.