Volvo v40 diesel starting problems

[foxhole]

I think the pre-heater [if thats what it's called] is failing, car starts from warm immediatley but if left a while has to turn over for 20 secs+ to start up, [regardless of air temp]. Have new glow plugs and fuel filter fitted.
Is it a simple diy job to change? if so what should I be asking for?

 

[Spark123]

When you first turn on the ignition are you getting 12v at each of the glowplugs?