it isn't the manufacturer who rates the fuel economy or emissions, but rather tha Department of Transport, who drive it on a rolling road following a pre-determined series of inputs. So your gripe is really with the Government, not the manufacturer.
I agree, there's no point complaining to the manufacturer, it's nothing to do with them and the figures quoted should always be taken as a guideline, not a guarantee of what you will obtain with normal driving.
He informed me that it is not up to the 50+ mpg of my last renault because it is the latest engine
That comment doesn't make sense. If it's the same fuel type/engine capacity, there's no example where an older engine is more fuel efficient than the latest engine.
If you can't have it both ways, no engine is hot on performance and economy at the same time
The latest diesel engines from manufacturers such as BMW and Mercedes are incredibly powerful, yet low on emissions and return superb fuel economy. Example: BMW 640d, 313hp. 0-62mph in 5.4 seconds, monstrous torque, yet only produces 144g/km CO2, meaning a mere £130/year to tax plus 51.4mpg on the COMBINED cycle! The only downside is the price but you can't argue about any compromise between power and economy with an engine like that!