A coincidence to read your thread, Oldmutton, I'm having a relaxing day after having a colonoscopy yesterday afternoon.
(3 weeks ago I had what is called a 'CT Colonoscopy virtual', after 2 days on a 'low residue diet' & the prep the day before, I went to the CT dept., at our local hospital at 8am, I was able to drive home at 11am.).
Yesterday, after 5 days on the 'low residue diet', the prep on Sunday evening & again at 6am yesterday.
I had my appointment at 3pm. The staff were kind & understanding of my worries.
For the procedure I had the sedation & the gas & air, I was fully awake & aware & could see the screen most of the time. Uncomfortable & slightly painful, but bearable.
They removed 2 polyps, also confirmed my (suspected) diverticular disease, they told me there was no visible signs of cancer, (which is why I'd had fairly swift appointments). This they will be able to be sure in a few weeks & let me know. I had already been told that I couldn't drive home, or for the next 24 hours. I was home about 6:30pm.
Last night I slept for 7-8 hours, (unheard of for me!).
I don't know what happened with the sedation, I feel more sleepy today than I did yesterday.!! 
Sorry if this is long & garbled!
In short, the prep is worse than the procedure itself, trust the medical staff, tell them if you are anxious. Easy to say, but try not to worry about the 'what ifs'.