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Thanks all, that worked.
[Edited]
Spoke too soon. It appears that the problem is some sort of clash with the combine class. I am combining a number of different documents and it appears that if the document is the first one with a tikz diagram it works fine but if there is another document that gets ...

I am using ktikz (on linux) to generate the following with transparency and it works but when I include it in a larger document and process it with pdflatex and view it with evince, there is no tranparency. The error from evince is "ExtGState 'pgf@ca0.5' is unknown" and Acrobat reader on Windows ...