You may find that in order to grow you business you have to start with trust. There is no guarantees that your staff or even yourself may not have an accident on the roads that will impact on you no claims. All you can do is hire as far as known to you what you consider to be good and responsible staff.
As a caution before giving staff vehicles it should be written in their contract terms or staff handbook the conditions and responsibilities of company vehicles. If vehicles are strictly for company use it should say so; even things such a paying for fuel who, how, reimbursements, maintenance, parking etc. Incorporate all these terms, go through it with them and get them to sign such an agreement.
I hope this helps.