Musht, thanks for the info, I agree with everything you've said. Unfortunately the problem isn't the process, but the file format.

I've got a mould housing and IM company ready to go, the only thing I don't have are the inserts. There are loads of engraving companies from Hounslow to Hong Kong who can engrave the detail.....as long as they are in IGES/solid format. I've also found investment casting companies who are happy to do a quote, all I have to do is....send them an IGES file :-) Same goes for spark erosion, etc. You can convert mesh files (stl, obj, etc) to solid in a number of packages, but only if they are less than 20,000 faces. Unfortuntely mine are bigger than this limit and I can't find anything that will do it - would love to be told otherwise!
I'm intrigued by your suggestion to bake an sls. Is that something you can do with SLA as well? I'd need the resolution of SLA as SLS is too grainy for me.

As you say, there is the option of a multi stage approach involving SLA then casting them, but that may be out of my reach. I thought it would be worth exploring the possibility that someone with their own set up might be set up for stls, so here I am ;-)