Londonclubguy,
You've got little to fear; this you should know before you strap in.
Clear air turbulence comes from several sources, but amounts to nothing more than ripples in a stream. Sometimes big ones,sometimes small, but turbulence is nothing more than air which isn't quite smooth. Think of it as a vehicle driving over a dirt road, or a boat on a river in which the current isn't glassy flat in appearance. The swirls and eddys and bumps you see are exactly what the atmosphere is doing when you experience clear air turbulence. The airplane won't simply "drop" or fall out from underneath you, and the wing is still producing the same lift it always did. It's merely going through a bumpy patch.
I've been flying professionally and privately for many years now, and have experienced all kinds of turbulence. Once in my entire career I've hit turbulence in which the bottom fell out, and it was a windshear in a thunderstorm...a place you won't go as a passenger on an airline flight. In my case, I was in a turbojet airplane penetrating thunderstorms as part of a weather research project, and we were in some very extreme weather conditions. You will never encounter such conditions as a passenger; during airline operations our primary job is to keep you a)safe, and b)comfortable and of course c)get you there on time. You'll arrive safely, and as comfortable as possible, and in as timely a manner as circumstance allows.
I've had a lot of nervous passengers over the years. Turbulence can be frightening if you don't understand it, or can't see or control it. As a passenger, you certainly can't see it (but you can feel it)...and obviously you can't control it, either. What you can do is understand it better, and that's simple.
The most common source of clear air turbulence is a wind shear which happens between the jet stream and the surrounding atmosphere. The jet stream is a river of air moving at a fast rate, high up in the atmosphere, It usually lays between layers of the atmosphere, in an area known as the tropopause. This is a place where the rate of air temperature change occurs. Simply put, you have air moving in two directions and two different speeds. Where they "rub" together, there's some turbulence, a little like water flowing over rocks in a stream. The amount of turbulence varies considerably, but most of the time it's little more than a shaking feeling, or sometimes a bouncing feeling. I find it hard to stay awake most of the time in chop and turbulence as a passenger, because it rocks me to sleep.
Sometimes the swirls and eddys of the atmosphere colliding can be a little stronger. In these cases air may be moving up or down, and may cause jolts or bumps for which you definitely want to be seat belted in. These are akin to perhaps white water rapids, by comparison. Still nothing to fear; the airplane may feel like it drops a little, but the sensation is misleading. The airplane is still held up by many tens of thousands of pounds of lift, still has good, solid airspeed, and will keep flying safely to the destination.
Airline aircraft are equipped with radar to see precipitation and moisture...which are often signposts of turbulence. Aircraft pass reports of turbulence to air traffic control, and to each other, and most of the time areas of turbulence are known and identified...and we slow down when passing through them, take different routes, or change altitudes.
Turbulence in an airliner is usually a non-event. I fly all over the world, and it's seldom I encounter turbulence in an airline environment. I've spent a lot of time in severe and extreme turbulence while working at very low altitudes in the mountains, and the chances of you experiencing anything remotely like that in an airline environment are almost nil. You should be able to approach your upcoming flight with confidence in both safety and comfort. Take your mind off your worries, and enjoy the flight.