Well, then... given that you have stored the camera in sub-freezing conditions (and, possibly - though I am not acquainted with summer in the UK - in conditions that get hotter than 95 degrees F. when the car is in the sun!), and given that the Canon Powershot series (and others) are sensitive to those temperature ranges, and also given that the camera is over 2 years old, I would say there are two things that are evident about the case:
1. the temperature ranges the camera has been subjected to over that period of time, have evidently ruined the image sensor. (This may be tested by previewing the picture you take, and seeing whether the preview also sometimes has the image defects you speak of. If so, that would evidently be an image sensor problem.)
(Cite: "32-104°F" is stated as the operating temperature range ... I'm surprised at the high end being stated as above 104, but that's what it says... in the camera's Specifications at http://www.usa.canon.com/cusa/support/consumer/digital_cameras/powershot_a_series/powershot_a1200_silver#Specifications
2. Since the camera is over 2 years old, since the date of purchase, any possibility that it might be still protected by warranty terms, are very unlikely at least if not impossible. Canon's usual warranty protection terms for cameras is one year (though longer terms are sold as 'Extended Service plans' sold with the camera itself.) And the temperatures plus the drop might void the warranty coverage in any case.
Therefore, unfortunately, the camera will need servicing, and such servicing would be at your expense.
The link to go to, to find out how much having your camera serviced by a UK firm (that is not Canon, evidently) would cost you, is here:http://uk-repair.com/repair-mobile/canon-repair/canon-powershot-a1200-repair/3419.html
with the square blue button lower down on the page, being the place to click to get a free estimate of the cost of repair.
If you wanted to send or take the camera to Canon UK themselves, on the other hand,http://www.canon.co.uk/support/consumer_products/services_locator/index.aspx
is where to go, to find out what service locations Canon UK has for your type of camera - select, under Product Group, the 'Digital Compact Camera' choice, and click on the Search button.
Note the 'Repair Process' link at the right, on this page... that tells how to send your camera to Canon UK for repair, and what would happen.
I'm sorry to have to pronounce this opinion, but the case is rather evident. (If you would prefer a chance at a second opinion, that might differ, from another Expert here at no extra charge, just let me know, and I will Opt Out of the case.)
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