Sold a car, broke within 10 days, not offered refund, claim petrol was poor quality

Bought an older BMW (2008) from a local car dealer in Surrey, England. Paid 4k, only had 85000 miles seemed good value. Broke down within 10 days of purchase - I had it towed back to dealer to look at. They now claiming that the petrol used to fill up the car wasn’t good enough quality. I made it really clear when buying the car that it was petrol and this was confirmed, no mention of any need for some sort of ‘special’ petrol. They said that the fuel station where I filled up may be at fault for a bad batch of petrol - this just seems ridiculous. I have said I want a refund for the car as it is clearly faulty, they are refusing - I’ve now got no money and no car - what can I do?