As always, many thanks for the observations and guidance. I think you have helped me avoid wasting much money.
I suspected the attachment offered by Chronos is only suitable for Bridgeports or Bridgeport clones. Even then I can't see how the dovetail support would fit onto the Bridgeports I have seen. The GTS item might go onto a Warco, although the taper is an issue. And there again, who would risk that weight of unsupported 'gubbins' hanging off the Warco spindle? Furthermore, as we all know, rigidity is 'all' in milling, and as has been pointed out, none of these set-ups, especially the GTS one, is going to be very rigid.
I don't think I can afford a horizontal mill or have sufficient work for one; neither do I really have the space.
However, I already have a three-slot rotary table that will configure vertically as well as horizontally, plus a dedicated three-jaw, self-centring chuck for it. If I bought another four slot rotary table (superior to the three) that came with indexing plates, I think I could contrive set-ups that would do everything I was hoping for with the horizontal 'attachment'. Yes?
Thanks again for all the valuable guidance.
Best Martyn