I am relaying this story on behalf of my father, who swore my mother and I to secrecy when it first happened because he was very scared by his experience and didn't want to be ridiculed if details ended up in the public domain. Over a year later, he's finally agreed to let me share details of this incident with MUFON because I've convinced him it should be recorded *someplace* publicly, even if he doesn't want to go public himself.
On May 1st 2006 my father had to drive from Chester, England to Porthmadog, North Wales, for work reasons. At approx 4:45 PM on the return journey, he was driving south, approaching the village of Gellilydan when he became aware of a "silhouetted cigar shaped object" flying 'gently' below cloud level in an easterly direction about two miles ahead of him. He lost sight of it a few times due to the landscape, but having passed by the village, it became apparent that it was flying slowly north (towards him). Assuming it was a 'regular aircraft' that he couldn't quite visualize properly, he was more interested than concerned at this point, and due to its apparent flight path over the open countryside, he figured it would clear him by less than a mile to the west when it passed. So, his interest was piqued ("is it a helicopter or a plane?" he wondered) and he reduced his driving speed to approx 20-30 mph in order to get a better view. 
As the flying object approached a point about 3/4 mile south west of his vehicle, he realized it was actually 'cylindrical'. It had a reflective convex base and a very subtle (barely noticeable) "electric blue" aura. He could only really see the underneath due to its angle. This took him back ("my heart rate accelerated") and having a camera on him, decided to pull over onto a grass verge and take a photo.
As he did this and scrambled for the camera, he realized the object had accelerated in his direction (it was very high up) and a huge shadow enveloped the area for about a mile around. "It was like a very dark thundercloud was right over me." He pushed his head forward in the windscreen and looked up (awkward angle) - and to his horror could see that this thing was about 1/4 mile wide and just hovering beneath cloud level directly above the part of the road he was on. He said it had huge tiles on its convex base. He had never seen anything like it in his life. It was "impossibly enormous". He realizes -- due to the sheer size of this thing -- that he probably underestimated how far away it was previously, but in relaying this event we've decided to use distances as he "perceived them" at the time.
Anyway, his heart was pounding, he was truly terrified because he couldn't make sense of the object and he was in the middle of nowhere (Snowdonia National Park) and decided to get the hell out of there. He hastily started up his vehicle and pulled off when he noticed the shadow just disappear, "like somebody flicked a switch and the thundercloud vanished".
He looked around as he drove, pushed his face into the windscreen and looked up but he could no longer see it. Then (within sixty seconds), as he passed the Trawsfynydd Twin Nuclear Reactors, he became aware of it hovering above some high ground to the west. The camera was on the passenger seat so without stopping the vehicle he just instinctively picked it up, snapped, then accelerated to 70 mph intending to get out of the area as fast as possible. He was overjoyed when he realized this 'object' had appeared on the exposure because it proved he wasn't going mad and he *had* actually witnessed something very unusual. If it hadn't, he says he's not sure he'd have told my mother and I the story in case we thought he was cracking up.
