Glow plugs don't usually make much difference to the starting on Hdi engines, yours of course could be the exeption of course. You will find them on the cylinder head below the injectors. There should be wires connecting them altogether, if you connect a voltmeter between any one of the glow plugs and earth, when you switch to the glowplug poition on the key you should have 10 - 11 volts for about 15 seconds, if you do the relay is OK but one or more of the plugs may have failed, if the voltage is 11 - 12 v that would indicate failed glowplugs.