The dealer can be held liable for any faults either present or developing on the date of sale for a period of six months. Given that the van is blowing smoke when it is going up hills, I’d say it’s highly likely that a fault is developing. This means the dealer is liable, explained here: www.honestjohn.co.uk/faq/consumer-rights/

There could be a number of causes for the blue smoke. It could be that some of the seals on the engine are on the way out, leaking diesel into the sump and oil. However, given that you’ve only just bought the van, I think it’s unacceptable for the dealer to ignore this and fob you off. I’d recommend taking it back and demanding that they investigate the problem and put it right. If they don’t want to do this, ask for a refund. Explain that the van is clearly developing a fault and they are legally responsible. If they refuse to help, have an independent inspection to determine the fault and get some legal advice. You can then take it to the small claims court, where you can claim up to £10,000 in England and Wales.