BTW, battery issues WTF ?? you can buy a no name battery from Halfords that comes with a 4 year guarantee for £70. If this piece of 16 month old scrap can not keep its original MB supplied battery charged what hope for it's future. Stop making excuses. what is happening to the OP is totally unacceptable.
OP get shot of this car now. Sale of goods act , now called 'Consumer rights act 2015' is the hammer you have to batter this dealer into submission. a friend of mine used it to reject a brand new £14K motorcycle. He got all of his money back.
If you took out a loan or used a credit card for any part of the purchase , even better.