I've never had to reject a car before, but I didn't think it was as simple as just saying you don't want it anymore? I was under the impression that the dealer/manufacturer had to be given a reasonable attempt to correct any warranty issues, but I might be wrong. I'm also amazed that another user on here has been told they will just have their car swapped for a new car - as pointed out already, the new car is probably already in the country or at least on it's way, so will likely have the same issue if it's widespread. Also, I can't see MG just giving out new cars as replacements, they'll soon run out of cars if the problem is a design fault.
I do understand your position though. Back in 2012, we bought my wife a Mazda CX-5 which at the time was a comlpletely new model with a new engine design. After a couple of months, the forums lit up with users noticing that the oil level was increasing and going above the max mark on the dipstick. Most of the discussions were around long term damage to the engine (even the manual specifically warned not to let the oil level go above the max mark). We had a lot of money tied up in the car so were understandably concerned that the engine wasn't going to last. Mazda dragged their feet, but eventually issued a recall including a new engine map and (I kid you not) a new dipstick with the max mark moved further up!
Despite all that, we did over 70k miles over four years before selling the car and it never put a foot wrong. We were so pleased with it, we replaced it with another CX-5. That one then had a safety recall for brakes and was off the road for a month waiting for parts! We did have a free hire car during that time, but it was still a pain.
I've also had recalls for a Ford Focus (handbrake may release unexpectedly) and my current Mercedes A250 (a sensor that effected economy).
It's a long way of saying that although it's worrying to have warranty issues so soon after getting the car (particularly mechanical ones) and the inconvenience that goes with it, we still don't really know what the cause or effect of the leak is. Until then, as others have suggested, document everything (including any out of pocket expenses) so at least you have evidence to fall back on. Only you can decide to try and reject or not, but if it were me I'd probably give it a few weeks until there's something official about the cause and then make a decision.