2006 2.0 D4D start up smoke (only after long runs)

[Munter]

So during the summer I noticed a few "lumpy" and smoky starts (a few seconds). Very similar to what I had with 1 or more glow plugs not working on a Vectra. But this was the summer and would do it on hot days. As time has passed it's become clear it only does it after a long run, and when cooled down. A short trip to the supermarket, and it'll start fine next time. A 2 hour+ run sit all day and next time it'll smoke/lumpy start.

Given it starts ok on cold mornings after a short run I don't think I have a dead glow plug. Just had it serviced but they found nothing wrong either. (Short run to the garage).

No significant change in oil use, and it's not blue smoke. Definitely looks like unburnt fuel smoke.

Any ideas?