Thanks for the replies everyone. I don't think the "No strings" shot is the best of the bunch, but it is kind of weird looking - good for a laugh 
I'm hoping to get some better light to have another go at shooting some more action sequences and really try to nail the focus and composition as well. This was my first time shooting a dog on the run and I didn't know what to expect in terms of focus performance from my 40D. To be honest I had concerns that it would underperform quite badly with the black dog as a target, but I'm very pleased with these results. I'm hoping I can get better results with more light and a lower ISO.
Here's another daft example....
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40D, 70-200 f/2.8 IS, 200mm, 1/1000, f/4, 1600 ISO. Processed in LR2 to adjust WB and exposure. Sharpening left at LR2 defaults.
and a more conventional shot....
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40D, 70-200 f/2.8 IS, 200mm, 1/2000, f/4, 800 ISO. Processed in LR2 to adjust WB and exposure. Sharpening left at LR2 defaults.