Good to hear that you found a solution.
Just a couple of comments.
>I can only assume that is due to downloading them on a PC and writing them to a CD for a trip over
Yes, the resource fork of the Macintosh file had apparently been lost/damaged. A self-expanding archive (.sea) file would normally act as an application on the Mac. If something has happened to the resource fork, the self-expanding capability would have been lost. The data fork would still be there, though (and in case of a StuffIt format .sea, it can be opened/decompressed by a working StuffIt Expander).
In order to protect Macintosh application files on the Internet, a BinHex (.hqx) or a MacBinary (.bin) encoding is used. The encoding can be seen as a shell around a Mac file. StuffIt Expander can be used for the decoding on a Macintosh computer.
A plain StuffIt compression (.sit) may work well for protection as well.
If an encoded file has been on a PC, one should not expect it to automatically open StuffIt Expander once on the Mac just by double-clicking. Instead, drag the encoded file onto the StuffIt Expander icon, or open it from within the utility.
Jan