1.5dci cold start issues

Hi all, my 1.5dci clio takes ages to start first thing in the morning or if left for a few hours, I would like to fit non return valve in the diesel line as I think the problem is due to syphoning back, if I pump the primer bulb first thing before starting can see loads of air bubbles in the line but priming makes starting easier, question is can anyone advise which diesel line the valve needs to be fitted, pictures and a how to do would be great

Many thanks

1.5dci cold start issues