(Reuters) - London stocks retreated on Wednesday as a spike in novel coronavirus cases across the globe compounded fears of a second wave of the pandemic, while builder Crest Nicholson tumbled after issuing a grim forecast for the year.The homebuilder fell 2.9% to the bottom of the FTSE 250 as it posted a first-half pretax loss and said it expected annual adjusted pretax profit to fall around 60% to 70% due to coronavirus-led disruptions.The blue-chip FTSE 100 was down 0.5% and the..