I’ve recently bought a used car on finance with a deposit of £1000. On day three of having the car (less than 50 miles later), the engine management light came on. I rang the dealership and took it in. They ran a diagnostics and the problem is with the diesel particulate filter (DPF) amongst other things. They hooked it up to a laptop and did a forced regeneration and the light went off. I took the car home and less than 50 miles later, the engine management light came back on again. I had another diagnostics done and it's coming up with the exact same problems as before. I’m well within my 30-day warranty, so does a dodgy DPF mean that the car isn’t ‘fit for purpose’, and where would I stand if I chose to reject the car?