Some well known retailers have clauses saying that if you have more than 2 repairs on an item it will be replaced - Dixons/PC World/Currys do this. You will however have to have the fault verified. Whether the second 'repair' constitutes part of that is up to them I suppose.
Either way you should have a pretty good reason to get a replacement. Discuss it with a manager of the store, providing all documentation you received regarding the problems. Under the Sale of Goods Act the item would be deemed of unsatisfactory quality (or 'unmerchantable' quality) if it required this many repairs in one year caused by manufacturing faults.
As usual the best advice is to approach this with resolving a problem in mind not blowing off steam at somebody, calmly explain the situation and you will find you should get a resolution.
A refund as you suggest would not be automatic as you are only entitled to a full refund in a reasonable period of time, or a partial refund after that.