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Definitely reject the car, in writing, explaining your reasons for doing so and pointing out that you bought the car from them, not the garage who did the MOT, so they're the ones who are responsible for ensuring that the car is of satisfactory quality and fit for purpose which, if it's unsafe to drive, it clearly isn't.
I doubt the dealer will back down - second hand car dealers aren't renowned for their humility, generosity or probity - so I'd second Dammit's suggestion of taking it up with your bank and seeking a chargeback.
If you can get something in writing from the garage that did the inspection, that would be most excellent too. Worth paying a bit for, if necessary - it might be the thing that sways the bank in your favour.
citizensadvice.org.uk tells me that the car can be rejected on the basis that it is not ‘of satisfactory quality’. This seems to tally with @Soul 's comment.
From what I understand of your knowledge and experience from posts on here, I would certainly appreciate your comment if you have one - thanks.