Help wanted - Three point seat belt fitting

Hello the CC! We are in the process of buying our first motor caravan - an Auto Sleeper Symphony. Although old, it's in good nick and well priced. It currently has 3 seats which have three point belts but the side bench seat has only one lap belt. We hope our daughter and family will use it but with two small children will need four full belts. It looks possible, but who could we get to do this work for us? Preferable somewhere in the Peterborough or south Lincs area.