Now running beautifully again - it turned out that it was the good old mounting gasket.
Despite not showing any change in revs when I sprayed the base with car cleaner - the usual method to test on the car I'm assured - the car was still missing badly once warmed up and under load.
On a recent test run a smooth driving with a small bit of choke worked for a while, then with zero or low choke I was able to provoke the problem with just a bit of firmer throttle action.
Armed with a new FEBI Bilstein gasket mount and a Weber service kit I headed to see Tim - with the new gasket available we were able to manipulate the existing gasket enough to reveal a split on the off-side.
When I got my Weber it had been on a car for a short time prior and didn't have a new gasket with it - this meant Rouge was still running the original 1985 factory gasket based on the part number I discovered yesterday so off that came and I took the opportunity to get the man to strip and service the carb.
Also made up a small bracket extension to keep the front of the airbox in a more secure position and less likely to put any stress on the new gasket.
Checked the fuelling - 2.74 so bang on.
Low and not so Slow Rouge is back - it has to be said I took the long way home (missing the start of the rugby) having not had a chance to really give the old car some beans since before November.