Who is to blame if a car sent in for repairs is stolen?

My vehicle was taken from the dealer’s workshop but it says it is not liable and wants me to claim on my insuranceTwo months after I took possession of a £40,000 factory-ordered VW Golf R, I was involved in an accident that was not my fault. It was taken for repair by the dealership I bought it from, Inchcape in Macclesfield, and was subsequently stolen, along with two other cars, from its workshop. Inchcape has refused to accept liability and advised me to claim through my insurer,