I'm with Mechanic on this. Have a few drinks, sleep it off and give them a call. Just calling them back to thank them for the opportunity will place you back in their minds, and if they need anyone in the near future (people dropping out, or future events etc) you'll have placed yourself much higher on that list.
And who knows, maybe they just had too many good candidates (likely, considering the employment situation in the UK) and couldn't take everyone on. I had that situation once, I was applying for a job, turned out about 20 of us were applying for 3 spaces. I didn't get it, and asking the manager about it afterwards he'd said that I was their number 4, and I'd only just been edged out. They later called me up to offer me the job (I guess one of them couldn't hack it), but I'd started working elsewhere by then.