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Thanks guys. I ended up using a combination of suggestions. Basically by pressing ctrl-t to get the free transform info open on the old object I can read off the centre coordinates, the width and the height. I then free transform the replacement object to the same specifications. It turns out that there's a bunch of rounding in the numbers displayed, but by adjusting them a fraction of a %/pixel at a time I eventually got everything to match up. Problem solved.