bad starting vw diesel 2.5 2003 turbo 88bhp?

All computer read outs are clear,new filter ,glow plugs perfect
The engine starts then dies straight away.A lot of winding by the starter motor usually gets it going but only just.Starting when hot is also very poor.No fuel leaks visable and she goes and pulls very well even fully loaded towing a caravan.A friend has suggested the ignition switch/barrel can cause problems.
Milage is 140,000 Its doing my head in any suggestions?