OK, it all makes sense now. I kinked the fitted fuel supply line going into the fuel distributor on removal (did not counterhold the female end until I realized what was happening). So the rough idle is cause by lack of fuel pressure in the fuel distributor caused by a narrowing.
Off to the junkyard... (with tools...)
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)