I have updated my iPhone and my daughters iTouch in the last week or so and it is a large download. The length of time it took to download is irrelevant and I cannot think why the AV and Firewall would have created a problem.
If you updated the Touch through iTunes and an Apple account then you should be able to reapply the update if, when it is returned from Apple, it has been reset to factory state.
Returning the device to John Lewis would only have resulted in them sending it to Apple so you have done the sensible thing in cutting out the middle man.
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