-13C was recorded at Liverpool Airport in December 1981, which is a mild location even by local standards. -9C was logged in December 2000 (though the Echo the next day had "-10C" emblazoned across the front page in large font). -15C at the airport would be truly exceptional. I dare say there's a good chance we had such temperatures in some of the severe cold spells of the past, such as February 1895 and January 1881 (when -18C was recorded just up the coast at Blackpool). Particularly interesting is the -10C logged in December 1995, as there was hardly any snow about (just lots of ice and frost). In contrast, the -13C in December 1981 was recorded under optimum conditions (deep snow cover, clear skies, no wind). I wonder how low it could have gotten with some decent snow on the ground.