Hi Tony,

It would be interesting to know how your mechanic knows it has been cracked a while, but regardless, you need to have a think about whether or not you're happy to hold onto the car. The one worry I'd have is how low the oil level was allowed to get previously, as that can cause permanent engine damage. If the seller is willing to take the car back and give you a refund, that's probably the best way to go, but they may also argue that the car can be fixed. See what they say.