To complete this thread.
It turned out the consortium doesn't have these, but Colin Clamp pointed me toward ebay.de, where a supplier sells two types of Flavia exhausts. These are IMASAF manufactured (I don't know anything about this operation, though quick Googling shows that they are Italian). I opted for the earlier type, which combines the flow from adjacent cylinders earlier than the later type. Illustrated below:
Early (1800);
Late (2000);
This Saturday I fitted the system, which arrived promptly, very well packed, and with extra clamps (though no manifold gaskets) supplied. Quality appears excellent and it went together without much trouble. I now know that you need to fit the rear of the system before the front, due to the passage of the system over the rear beam axle. Finding this out meant fitting the manifolds twice! I can tell you they are a lot harder to fit when they have the whole of the rest of the system attached...
Justin was right about the rectangular plate being the culprit for the noise. A simple task to repair, but I'm very happy with the more muted sound that the new mild steel system produces, so if someone wants to make me a nominal offer for the stainless system, which is in good condition apart from the separated reinforcing plate (an easy repair if you have the right welding kit for Inox), let me know.
Link to ebay item for anyone interested.
http://www.ebay.de/itm/Auspuff-Set-Lancia-FLAVIA-Coupe-1-8-li-re-Abgasrohr-Mittelrohr-MSD-ESD-/380858503271?ssPageName=ADME:L:OC:US:3160