It's possibly a false widow - but don't go killing them just because it's the latest Daily Mail craze. They do not go out of their way to bite you.
If you've seen it, it won't bite you - because you've seen it - the only insect that bit you is the one you DIDN'T see, and by then it's too late.
I went for a slash in the undergrowth of a layby on the way to the British Grand Prix this summer, got bitten on the leg by a single solitary midge and suffered for about 2 months afterwards - ended up going to the doctor and getting antibiotics, which didn't work. So do I kill all midges? No way - they're just God's creatures. It was the fetid human @*!# he'd been feeding on that made me ill, so I'm going back to that layby with a gun, ready for the next bastard who tries to take a crap in that undergrowth. If you know what I mean.
If you're extremely unlucky and you get bitten, 99.95% of the time it's just a little insect bite - so be brave. And of course, like all spiders, they do good.
Ooooer - I just went off on one.