It would be worth far more in parts than selling as a whole car. If you do sell it whole you'd have to price it on the basis it needs a new engine.
Regarding the new hg and now it's failed again(i presume from contaminated oil), if you had it done professionally at a workshop you could go back to them for advice. Have a look at the sales of goods and services act for more information but in very basic forms any work completed needs to last a reasonable length of time. 12 months isn't long enough dependant on mileage. If you feel hard done by and the act helps you, get in contact with trading standards. If you done it yourself or a mate unfortunately it's tough potatoes.