Suggest you put a magnet on the sill near that crack. If it falls off, grind the underseal off around the crack and find out if you have good metal around the crack. A crack in good metal just there would be unusual and easily repairable, but if the sill has rusted away just ahead of the wheel arch, a known problem area if the hood drain is blocked,and someone has repaired the rust with plastic filler then a crack is quite possible over time, and will probably lead to an MoT failure