I would contact the site owners and building managers and advise you wish to claim from them. Advise that they are 'vicariously liable' for their contractors while they carry out their activities and have been negligent in their actions by not having resources in place that would allow their traffic to access their site from an alternative entrance. Without proof of who has damaged the vehicles, unless you have on dashcam or CCTV, you may well struggle and may need to claim via the Motor Insurance Bureau, and make a claim for an untraced driver. I would hazard a guess that the vehicles have been hit by an HGV and trailer, and may not have even known who has done it. Do you know the date the cars where damaged? The site should know what HGV's were coming on-site and making deliveries, it should be straightforward for them to assist in establishing.