Please help where do I stand.

GAZZA544

I traded in my ford mondeo auto which they gave me 500 for and 350 cash for a Vauxhall astra estate manual. I am struggling with the manual as I have arthritis and put it on ebay to get rid of it and get another auto. When I test drove the car on the 18/02 I noticed vapour coming from the exhaust but put it down to the engine being cold. 1 week later I noticed my heaters would not get warm and found I had no coolant so I refilled it.5 days later I noticed a leak coming from the top hose on the radiator and replaced it and topped up the coolant. 1 week on and it is still leaking slowly apparently coming from the thermostat housing which I told the buyer from ebay who has just come to have a look at it he said that was fine he could fix it but he didn't want a car with a head gasket failure. This car was sold to me for 850 with a head gasket failure from a used car dealer do I have any rights after nearly 3 weeks.