after a lot of scratching heads on trying to find out why it was misfiring i decided to take the 172 to a local renault specialist. Plugged it into a clip computer and the only error shown was the misfire. The guy working there did some scatching of heads and came up with the conclusion that there was an airleak in the inlet manifold. He squerted some liquid towards the inlet manifold gasket and tada! there was the problem! so today the lower manifold gasket was changed along side the injectors and the lower manifold.
It now runs very very smooth and changes gear like a dream, what a great car
/carl