hi stu, i had one which started off just like yours, then started cutting out also, when you turned the key the immobiliser light went out then when cranking came back on. would initially (until the problem worsened) start. turned out to be a broken wire from injector pump immobiliser just before the loom runs up onto the o/s/f inner wing area, insulation had broken and water had got to wires and corroded them out (seems common on renaults this sort of age!)