The Motor Ombudsman is an independent body that handles disputes between consumers and motor industry businesses, such as dealers, garages and warranty providers, once you've tried to resolve the issue directly with the business first.
It's worth knowing this isn't universal: it only covers businesses that have actually signed up to one of its codes of practice. Not every dealer or garage is a member, so it's worth checking whether the specific business you're dealing with is covered before assuming this route is available to you.