Glow plugs
 

Hi does any one know if you have to change the glow plugs control unit when you change the glow plugs and is it as easy as unscrewing the old ones and replacing them

What did the car's error codes indicate? Assuming that (most of the time) that one or more of the glow plugs have failed over time - you replace (only) the failed glow plug(s).

The control unit on the M57 isn't (to my knowledge) selective - it warms all of them when the engine temp is below the threshold value (which escapes me as I write).