Renault Clio leaking coolant and juddering when idle

A week ago I noticed my car started to over heat on the way to work. I am conscious of this problem as I have replaced the water pump about 18 months ago. When I got to work I realised that the water and coolant in my radiator expansion tank was cronically low. I filled up again.

On the way home i noticed my car was juddering when stopped at traffic lights and roundabouts. It is quite a server juddering which has never happened before. Two days later my car was out of coolant again. I bought some radwell and added this to the coolant i put in my car. A day later the coolant was all gone again.

Initally I could not locate a leak, but I have noticed that the side of my expansion tank is wet and there was a puddle under the expansion tank once i had left the car in one place for a few days. I have checked the oil and there is no water in it and there is not oil in my water. There is no smoke either.

Could someone please explain to me what this could be?? I have people telling em it's the head gasket and then others saying there is no way. I have stopped driving the car, but I need to get it running again and without a diagnosis no mechanic will help.

Thanks. :)