The U.K.’s top law firms have improved their profit margin for the first time in years, as they continue to seek out efficiency gains amid a continued rise in staff costs.

An in-depth analysis of limited liability partnership (LLP) accounts for the 2017-18 financial year, carried out in conjunction with professional services firm Smith & Williamson, found the profit margin for the combined U.K. top 50 inched up 0.4% to reach 31%.