Is my business liable if a subcontracted driver causes an accident?

It depends on the nature of the relationship and the circumstances. Generally, a business is not vicariously liable for the negligence of a genuinely independent contractor, unlike for its own employees, but there are exceptions, for example where the business exercised a high degree of control over how the work was performed, effectively making the arrangement one of employment in substance despite the label, or where the business was itself negligent, for example in selecting an unsuitable contractor or requiring an unsafe schedule. The Health and Safety at Work Act 2015 also imposes duties on a person conducting a business or undertaking to manage risks to others affected by the work, which can extend to contracted drivers and third parties. Review your contracts for indemnity and insurance requirements, ensure contractors carry appropriate insurance, and seek legal advice about your risk exposure in this specific incident.