Further to my post last night, I can confirm that I too have an oil leak on my 3 day old car.
I have sent an email to the dealer (and copied the text onto the MG customer services email contact site). I have told them that I will reject the car and I have asked them to pick it up. I have also told them that this is non-negotiable.
A new car should be of such quality that it doesn't leak oil, a new car should also be fit for purpose, this is neither.
I have better things to do than get it repaired. I expect they will want to discuss it, but I am not going to.