They have to fix it or return your money. The dealer can be held liable for any faults that occur within the first six months of ownership. This is because the law assumes that the fault was either present or developing at the time of sale. Explained here: www.honestjohn.co.uk/faq/consumer-rights/

Return the van and tell the dealer they can either repair the fault or refund your cash. If they refuse either of those options then get some legal advice.