Activate product on Steam

Hi all

I bought Sins of a Solar Empire several years ago, before it was available on Steam.

I recently had to buy a new PC, and wanted to add it to my Steam library, but I get an 'invalid CD Key' response.  The product key is still available on my account on these forums - so I know its still (technically) valid, and as I am copying and pasting it, I can't be mis-typing it either).

Does anyone have any ideas short of buying the whole thing again?

Thanks

Urake