Linyphia triangularis

With about 670 British species, identifying spiders isn't the easiest thing, but it gets easier when you know the tricks - in the case of Linyphia triangularis, the black "tuning fork" marking on the carapace. 

Sony ILCE-7RM2
Canon MP-E 65mm F2.8 G SSM
f9 1/160 ISO 100