Adding this for anyone that comes across the same issue.

Noticed a slightly extended crank a day or so ago, along with a few seconds worth of poor idle and then ran as normal.

Checked for codes, and none showing.

Checked plugs when I got home again and all ok.

Checked the lpfp count and was at or around 50% at idle. No hesitation or misfires and none detected on scanner.

Rail holding pressure within parameters and both lpfp and hpfp showing requested and actual pressures where they should be.

Checked the plugs again this morning on a cold engine for damp, and they were dry and showing no signs for concern.

And now for the culprit.

On a stone cold engine, the engine coolant temp sensor was showing 80c while the radiator outlet sensor showed about 1c within ambient air temp.

A new coolant temp sensor now fitted and all showing back within normal tolerance levels and no more extended crank issue.

Just thought this might be useful for anyone with the same problem now or in future and hopefully it saves them time and money.

Good. That you are all sorted. Just out of interest, What is the ambient temperature you are talking about here for Cold Starts? Are you running on E10 or E5 fuel? How old or long / miles have the plugs been in. Did you measure the gap the plugs have when they were out.