Trouble starting...sometimes

[Adamskiking]

My van has trouble starting sometimes. Everytime I take it to the garage it starts first time no problem so they've ruled out the starter motor the garage started it 20 times in a row and every time it just starts 1st time so they say it's fine, I drive home, park it up and just test it and it really struggles again! it always starts in the end but maybe after a few times - it turns over but just does'nt seem to fire up. If anyone has any idea it would be greatfully received as I'm starting to look like a bit of a duffus at the garage!
Its a Diesel 4WD, 2.5lt 2002.

[Ian]

Have you got an immobiliser fitted? Sometimes these can interfere with the ignition sequence.

[Adamskiking]

I have got a Viper immobilisor fitted (that already was fitted in japan) we don't really use it as the fob is broken.
Should I get this taken out or replaced? would a english thatchman cat1 cause the same problem?

[Driver+Passengers]

Is it starting ok when warmed up and not starting well when cold? Failed/failing glowplugs might cause this.

[Adamskiking]

there really seems no rhyme or reason to it. Sometimes it totally fine and then it won't start. Might change the glow plugs anyway...is that an easy job or a garage job?