Argos is now listing the Marvel sets on its website, although you might have difficulty finding them. It's almost as if they don't want to sell them: they are not listed when you click on the Super Heroes image on their LEGO page but if you click on that link then search for 'LEGO Super Heroes' you'll find them. (thanks Obiron17)
Some branches of TRU are also selling them, although the outer boxes are apparently marked 'not to be sold until April 8th' so you might not find them in your local one. (from the forum)
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