Thanks for your help, everyone.
I once encountered poor-ish visibility during a lesson, about a couple of months back, around the Edinburgh area. Can't remember the exact weather but the EDI METAR was CAVOK (think there was a high overcast at what looked like 10000 feet. Vis. at ground level looked fine but climbing up through 1000' the vis. deteriorated rapidly to something more like 7-8k. Was quite alarming at first. Luckily that day's lesson was low-level Nav techniques! Vis. was much better out towards East Lothian but around Edinburgh the vis. was mince. Interesting experience though. As you say FL it was alarming at first but as long as you keep situational awareness it is okay. Not sure how I'd cope with 3k however! I can see how people can get lost very easily because all it takes is a moments distraction to lose your situational awareness and you are goosed!
Smithy