Paste in place isn't working??

Hello, I wonder if anyone could give me any pointers - or tell me what you need to know in order to help..

I have opened 2 files in CS5, a view of a house from the garden, and a blue sky to replace the grey one with. After making a selection of the grey sky (I managed that much today), when I attempt to paste it in place, it is either covering the original image, or showing through in lots of places I didn't want it (and not in my selection)

My fear is that I've messed with my settings, checked a wrong box or something. I'm used to the sky filling my selection, and being able to move it around. I've made this work for me a hundred times, but not today  I'm tearing my hair out!

Any ideas?

Much appreciated.

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http://www.flickr.com/photos/8189967@N04/