You have protection under the law no matter what the car cost.
Try the following: send a special delivery letter to the garage stating what is wrong with it and that you "reject the car in law, it is not fit for purpose, and you want a full refund". Give them a week in which to respond and say that if they don't, you will take legal action. If the garage fails to respond within the time given, go to your local county court and take out a summons (it is very cheap, and you get the cost back if you win). Don't worry about this - it is a very simple business and you sit in a room with a full blown judge (in a suit) who decides which of you is right and which is wrong. He is empowered to make a judgement against the garage, and no business likes this because it affects their trading status. Often these people will settle out of court. I did exactly this in exactly the same circumstances and won - plus costs! Good luck.