I guess it depends what you paid for the car but you should probably think carefully about what you hope to achieve.
Although you have the right to have "major mechanical issues" dealt with, you don't have the right to buy a car for a couple of hundred quid and then expect a seller to do a thousand quid's worth of work to get it into good shape.
In that case, chances are the seller will just refund you and then sell it to somebody with more realistic expectations instead.
If you did pay a lot of money for it, on the assumption it'll be in first-rate condition, it's more likely the seller will be willing to either fix it or, at least, refund you something to cover the expected costs.
Course, you should think carefully about accepting an offer like that too.
If you suggest it needs a gearbox oil-change and the seller offers you £50 to cover the cost, it's going to be awkward if it turns out it needs a new gearbox that's going to cost £600 to fit.