I've brought a car from a dealer and after a couple of days

I've brought a car from a dealer and after a couple of days noticed that the gear box kept crunching when I went in first gear, also that when I turned a corner there was a clicking noise. Also when I started the car the inside of the car smelt very strongly of exhaust fumes. My mechanic was on holiday but as soon as he returned home I asked him if he could check these things out. My mechanic only needed to listen to the noises that the car was then making and he told me that the car needed a new gearbox, a new cv joint and the exhaust seeing to as it was blowing. I called the car dealer (best cars) on the 13th day of receiving the car telling him that the car he had sold me was not fit for purpose and that I wanted my money returned. He told me that my mechanic was lying to me to make money from me and told me to bring the car back so his mechanic could take a look.
I decided to take my car to another mechanic and was told again that the gear box needed to be renewed as did the cv joint but I forgot to mention the exhaust. I then returned the car to the dealer telling him that I no longer wanted the car as I feel he was dishonest in the sale so I could not trust that he would do the repair to the standard I had expected. I also said that I had paid book price for the car and had not made a deal on
a defective car to repair.
The price I paid was £3,499 on a 58 plate Nissan Qashqai. I also asked if he could then communicate with my friend ( I had gone down with flu and couldn't cope by then.
My friend Amanda Wade then emailed him to say I was wanting my money back as the car was not fit for purpose and that she was now acting on my behalf.
The next day he tried to call me, I couldn't take the call so text my friend and asked her to call him. She spoke with best cars and they told her that they had changed the clutch and the cv joint and that they were giving me 30 days notice under section 7 to pick the car up as I owned it.
What do I do now?
As far as I'm concerned they haven't even repaired the major thing why I retuned the car. I purchased the car on the 29th March and informed him on the11th of April that I wanted to return the car and returned it on the12th April. When I spoke to my mechanic he said he didn't believe all that he said he had done, could have been done in that short time, even though the clutch didn't even need doing.
Please help me!!!