Did it last year – best event I've done in years.
You're only timed on the "special stages" which were all downhills. You just had to get to the top of the next one before your start time, or the clock started without you. Very relaxed atmosphere and plenty of time between stages to allow you to get to the next without going mad.
And no you don't have to be a DH god – a 4-5" travel bike seemed spot on for the event. The qualifying stage on the first day sorts out your start times for the main event so you end up racing between riders of a similar ability for the most part.
They're not running a stage at Kielder this year though are they – only Ae and Glantress according to the website?