Also, try Metposts. available at b+q etc. they do a 'bolt-down' post-holder, which is basically a metal plate, about 8"square, welded to a square socket of 2",3", or 4" to take the relevant sized posts.
If you haven't already bought the timber, remember to get it from a proper timber/builders merchants, rather than a diy shed; they tend to be much cheaper.
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