User1314 said:
Should I contact the police then, as Magnatom suggests? I didn't get the reg number of the HGV.
I would, will do no harm. This chap might not have anything to do with the incident at all, but he may have. The police often ask for witnesses who were around at or near the time of the incident.
As Landslide suggests, it may be that this chap was a witness and hasn't thought of contacting the police. You could help by passing on his details.
If you do contact them, let us know how you get on.