By Newsbreak Producer Matthew Veeran - Extremely cold weather is forecast to tighten its grip in many parts of the United Kingdom this week, with sub-zero temperatures plunging even lower than during the heavy snowfall of the past weekend. Weather forecasters predict the coldest nights of the year so far being last night and this coming evening, with temperatures expected to fall as low as -20C in some areas. A series of yellow weather warnings covering the next few days have already become active - with the latest warning over ice in parts of northern Wales, as well as areas in central and northern England. This report was compiled with assistance from the BBC...