I had almost the identical thing happen to me ten years ago.
I was taking my eldest daughter to plymouth Uni for her first term. Just past Bristol, a motorway service lorry hit a bump up ahead and a shower of gravel came off the back.
Some bits came towards me, like it was slow-motion, and hit the leading edge and top of the bonnet. I knew there would be damage so we grabbed the contractors details off of the lorry.
When I got Home the next day I rang the contractor.
Initialy they denied responsability, they said "people tried it on all the time".
I told them if they did not settle my claim I would go directly to the department of transport and make a claim from them for using a contractor that operated with unsafe loads!! The likely outcome of which could be a major motorway accident.
Probably fearing the loss of their big contract they paid for a front end respray by a top class bodyshop.
So if you can remember the company that opperated the lorry it would be worth a try.
Tim
PS I had no Reg number