This is a species of harvestman - like true spiders these belong to the class Arachnids, but are in a separate order called Opiliones; true spiders are in the order Araneae. Unlike true spides they don't produce venom or have fangs, and don't make silk. A pair of eyes is situated on a little blob on the top of the body, rather than an arrangement of 3 or 4 pairs at the front. They mature in late summer so get noticed more at this part of the year.
There are a couple of dozen or so UK species, with various body shape and leg length variations between them.
I'm no expert, and I'm happy to be corrected but superficially this looks like Odiellus spinosus.
Yep, that's the beastie, thank you! The markings on the back are less apparent and the markings on the tail more apparent than some of the other photos out there, but once you'd said Odiellus spinosus its obvious from a quick Google image search.