Automatic gearbox problem astra 1.8 4 gear

we have a vauxall astra design 1.8 4 gear automatic year 2007 and one day it just decided not to drive when we got it to a garage they said the radiator had crashed through the gearbox and it would need a complete new one which would cost us £2500 we've only had the car for a year. We think it was faulty when we bought it but the car dealer has since shut down. it has also had alot of problems like leaking transmission fluid and the AA had to come out to us one time and they said there was something seriously wrong with the car but it drove after this then cut out. We dont really know what to do as we really think the car was faulty, could we prove this? if yes then how? or is there a common fault with these gearboxes?