The website Broadcast collects viewer statistics in the UK and published this article a few days ago. According to it, The Valhalla Murders achieved BBC4's best Saturday drama result in two years:
"BBC4's Icelandic Drama The Valhalla Murders began on Saturday night with 800,100/4% at 9pm - the highest BBC4 drama of the week and the channel's best Saturday night drama rating in at least two years."