Normally I'd use loose-pin hinges to attached baseboards together, but when I fitted them to the Selly Oak boards, for some reason we ended up with a 2mm gap. No idea why, but it wasn't good enough. Maybe fitting a hinge in the corner between the boards didn't work. Or maybe I'm just rubbish.
Anyway, it was back down to the hardware shop for bolts. This isn't so bad as the boards come with alignment thingies - you can see the spike in the photo.
Bolts are to fit a 7mm diameter hole, to match the spikes, and I bought wing-nuts and the biggest washers they sell as well. The gap is gone.