Difficult to start

[PhoenixPhil]

i have a 1999 tdi with 200k on the clock. When i try to start the car in the morning it fires once but doesn't start. However if i keep trying it eventually it will go. I have fitted a new set of glow plugs and it hasn't made a difference. When weather is warmer don't have problem getting it going.
Any ideas anyone.

[Anthony]

check the wire harness to the glow plugs , its prone to fail

[Steve_PD]

Might be a weak battery

[PhoenixPhil]

Thanks Guys,
I've checked the battery - well actually it's recently been replaced. I've given the wire harness a look at and can't see splits or breaks. However i did notice that there is a rubber tube that links between the top of each injector and one of them is perished. What are these tubes for?
I've also discovered that there is a relay that controls the heater plugs that can cause starting problems. Where is the relay?
Appreciate your help