>Does anyone know of a product that can read a data file and advise the format?

I haven't used it, but others have used this product in similar situations (used to be called DataJunction): http://www.pervasive.com/dataintegrator/index.asp

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>I have a potential client that has data locked up in an existing system, and I only get the sale if I can read his existing data.

Are you sure it's legal to do so? A far better option is to get help from the company that created their current system.

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>The data is arranged in a number of files (like free tables), there is some ASCII and some binary data in each (although I can not see that there is a header using HexEdit, but I assume there must be). I can not see a DBC equivilant but there are .IDX files which I presume (and naming would agree) are index files.
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>The file extention is meaningless as they have come up with their own.