Could be anything really Paul,
If you have the ability to scan it then that’s worth doing as a first step, otherwise if it’s only manifesting the one time from a cold start the only other thing I’d try is running some fuel cleaning additive through it, or some tanks of Shell V-Power, or as I do already on my Touran (and will when Merlin returns to being my daily driver), running the Millers diesel fuel additive - any of which will give your fuel system a nice spring clean just in case it’s related to that.
Otherwise I wouldn’t worry about it too much but would just monitor to see if it gets worse.
EDIT - I see Paul replied along broadly similar lines (fuel system) while I was typing.