Changed the glow plugs on my 1.6 n/a d and found that the new plugs pulled to much current and where heating the wire and so not working ie not heating the engine properly.
Have a look at the wire going to the glow plugs see if its in ok nick as this seemed to cure the starting problems similar to what you are experiencing.
Used the cold start two times on my van when the battery was almost flat
good luck
Jon