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Hi Grant,
Extension tubes are useful bits of kit in the macro world. But, (and there's always a but) you need to have a clear understanding of what they are and what you can expect.
Principally, when you focus your lens to it's minimum focus point you will have screwed it out so the lens assembly is furthest from the sensor plane. An extension tube is just an empty tube that fits between the lens and the camera body enabling you to focus on something even closer than before. Extension rings normally come in sets of 3 in the ratio of 1:2:3; Ring 1 is the narrowest. Ring 2 is twice the width and Ring 3 3 times the width of N0.1. So you would use them singly, or in combinations of 1; 2; 3; 3+1; 3+2 & 3+2+1
This is where the price difference comes in. The £5 set you're looking at are purely manual, there will be no electronic connection between the lens and the camera. This means that everything will have to be done manually; Lens focus & Aperture setting. Your lens may not allow you to do this (check before you do anything else). The other hiccup is that when you've stopped down to the required aperture it will get a bit dark in the viewfinder and you may not be able to see what's going on.
The more expensive ring sets have the required connections and your lenses will function as they do normally, except you'll be focusing a lot closer. It may sound like a huge leap in expenditure, but depending on your lens capability it may be your only choice - it will certainly be your better choice.
There's a lot been said about the 'quality' of the cheaper brands. But to be honest there's no optics involved and the only tasks they have to perform is: 1. Securely fix to lens and body. You don't want your lens to fall off. 2. Adequately extend the electrical connections. 3. Don't let any stray light in. If they can meet those criteria they'll be no better and no worse than the branded Canon tubes.
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