As I understand it then, to make use of the back auto focus button, I need to via the custom function setup, disable the front autofocus button and just leave it as shutter button only, is that correct? Otherwise when I go to take the shot the front button overwrites the back button and goes through the focusing all over again?
Edit:
The lens I have are as follows,
1/ Sigma 18-250mm F3.5-6.3 DC Macro OS with SLD glass
2/ Canon EF 70-300mm F4-5.6 IS USM
I'd love to get some L series Glass Lens but the cost is prohibitive at the moment and those that might just be approaching affordable are usually shot to buggery as they tend to be heavily used.
Pat covered the setup nicely, so I won't repeat his comments - but yes, the metering and shutter release stay on the shutter button, and the camera only focuses when you tell it to with the rear button. It doesn't help much if you're not in AI Servo, though, because the camera will still behave as though it's in single shot focus whether you use the front or back button. You don't get the 'beep' (even if enabled) when using AI Servo, though, so other than the viewfinder image and lens motion you will not get a notice that it's in focus - AI Servo is focusing constantly and never stops till you release the shutter. That's good for moving objects, because a fast-moving subject will continue to be tracked by the camera right up until you shoot rather than stopping once it 'thinks' you're in focus. Apologies if you're already familiar with Servo, but some reading this might not be aware of the difference.
However, due to this continuous process, some people are frustrated by the shutter releasing when slightly out of focus. Custom function III-2 lets you tune it so that the camera will prioritize speed over focus, or vice versa. It depends on your shooting style and situations which you'll prefer. Again, it's a process of learning what works best for you and either adapting your behavior or tweaking settings until you get what you want.
Oh, also - some camera stores and places like LensRentals have certified used lenses for a good discount. That's where I got my first L glass and it was not only a bargain but reliable.