An update: I seem to have solved the problem! While pottering around (as you do) under the car, I noticed that the stub of the exhaust pipe - the short piece from the rere silencer to the back of the car - was indeed VERY short. It doesn't even clear the bodywork. I rummaged in the workshop and found one of those boy-racer chrome exhaust tailpipe extensions. Once fitted, the smells seems to have abated. I'll work on welding on a less flashy solution this weekend.
Thanks to all who inputted!
Now it's marked as Solved!
Thanks to all who inputted!
Now it's marked as Solved!