barca, use the search facility and you will find a wealth of information on this subject and much excellent advice.
I am one of many who have successfully rejected my Cooper after about 4000miles, after suffering many minor faults and then a large electrical one. It can be done relatively easily if you are prepared to stick to your guns.
Talk to your dealer about rejecting and if they aren't very helpful, call MINI customer services, or email
nick.sharman@bmw.co.uk (an area customer services manager).