Starting issue

[lightning]

My 2008 Connect TDCi has always started within a second of turning the key, even in temperatures of -15 degrees C a couple of years ago.

However it has recently been sometimes taking 5 seconds of cranking to start the engine. But this only occurs the first time in the day that I start it.
Some days it starts straightaway and others a fair few seconds of cranking is needed.
After that it always starts immediately for the rest of the day, even if it's been sat for a couple of hours or more.

What is causing this and is it the start of problems (injectors? fuel pump? glow plugs?)

The van has done 55,000 miles.