I put my mate's identical lens on and there was a vast improvement. I fitted the camera to the tripod and checked with the focus test chart as mentioned by Keith T earlier, and my mate's lens was only about 4mm out. (To the back). I hope I can assume it's more likely to be more the lens that is out, rather than camera or both. As mentioned earlier, it could still be both.
I tried contacting three different repairers, including Canon Australia, and none of them would even give me an estimate on the cost of recalibration! I find that very annoying. How is anyone supposed to make a judgement on whether to send it in or write it off and upgrade? They want between $30 and $65 to look at them and that cost will come off the price of repair. I've no idea what that price might be, has anyone had it done? The repairers all say "it depends what needs doing" Surely a ballpark figure could be given?
Sorry, end of rant.
Of the 70-300mm lenses, the Tamron is miles ahead of the Sigma. It'll be a bit more expensive but it's one of the best 70-300mm lenses on the market at the moment (barring the Canon L or DO models)
The Tamron SP 70-300mm F/4-5.6 Di VC USD. arrived today and I wasted no time in trying it out. Wow! Chris thanks for a great tip. It's fast and crisp and very accurate in the focusing department. I doubt I'll ever use the Kit zoom again. I'm saving for the "Nifty Fifty" now. Thanks everyone for your helpful input.