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Removing jpeg artefacts

A client has supplied a jpeg image that needs resizing from 670 pixels high to 1280 pixels high and converting to B&W. The problem I have is it's a jpeg landscape with dark mountains and a light sky and this boundary has noticeable jpeg artefacts which are only made to look worse when resizing.
I don't normally work with jpegs and the original files aren't available, so I have to make do with what I've got. Does anyone have any experience of removing artefacts? A tool / piece of software would be ideal as I also don't have much time to work on the image. I've got PS 2 at my disposal.
Thanks.
I don't normally work with jpegs and the original files aren't available, so I have to make do with what I've got. Does anyone have any experience of removing artefacts? A tool / piece of software would be ideal as I also don't have much time to work on the image. I've got PS 2 at my disposal.
Thanks.

Have you tried Genuine Fractals from OnOne. It very rare I use it but when I do it does an excellent job. Here's their website: here