Ford Focus engine cuts out

ok, engine was cutting out several months ago, changed fuel filter(thought i might have got some dirty deisel) seamed to have made the problem 100% better. i was driving from hereford to forest of dean yesterday in some pritty heavy rain. all was ok when i was getting back to hereford the car started cutting out again and the glow plug light starts to come on. pull up at lights , junction etc engine idles at just under 1ooo rpm then when you accelorate the car accelorates then just dies and the glow pug light comes on. start it back up and pulls off then the same problem when i have to stop again. any ideas guys/gals??????????