If you are going to replace the glow plug, also change the wire Glow Pin Screen. This is a wire sleeve that slides into the hole where the glow plug screws in. If this pin screen is damaged or burnt out, a new glow plug wont stop it smoking. This part can be ordered from butler technik, part number is 252121990113. I have changed my glow plug 4 times in 8 years and didnt even know this sleeve was there, a small bladed screwdriver can be used to remove once the glow plug is out. Cost is £18 with next day delivery.