2nd hand car dealership issues

[Kathr20]

I purchased a 2nd hand car in june 2019 a from a car dealer a Hyundai i30 2009. I checked the current mot it had failed on a few things but a day later work had been done. It had passed with not even a advisory. A month after purchase i had taken the car back as the back breaks were knocking as as i had seen on the mot it had failed on discs and was told new ones were put on the car and wanted them checked. They had the car for a few days when i had it back he said back brake discs and pads have been replaced. All was fine the knocking started again now and again but i assumed it was nothing. New mot today and was in for a shock!!!!! My macanic wasn't happy he said the rear sub-frame was in shocking condition he said theres no way that the damage wear and tare would of been caused in the time frame that I've had the car and should of shown on the last mot at least!!! He put the car up and showed me another car and what a decent sub- frame looked like with a little wear and tear. He then mine there are big holes in areas on my cars rear sub frame. He told be to get the car dealer to pick the car up immediately he wasnt happy for me to drive it to them. On top of this the break discs are significantly and obviously worn on offside and nearside. He checked the previous mot and said its just not possible for new break discs to wear in this time frame especially as ive only done just over 2000 miles in this time frame. He has said that i have been sold a car that isnt road worthy. I feel like a complete fool i felt safe purchasing this car from this dealership as my partner is friends with the man with a clean mot i thought that we would be fine. The man we purchased the car of agreed that the rear sub frame is in very bad condition but wants his macanic to check it himself because they wouldn't let a car go out that wasn't in great condition. Is my macanic right that while wear and tear will happen theres no way that amount of damage would or should happen in the time frame i have had the car, with the sub frame and rear break discs my car is now 2 months out of warrenty. My mechanic said if they dont resolve the issues then i should go to training standards he had 4 employees they all agreed that this is not typical wear and tear and these issues would have shown on last mot at least but there is nothing on 2019 mot regarding the sub frame issues. Can the car sales refuse to sort these issues out as the opinion of multiple experienced mechanics dont agree that its genral wear and tare