Purchase issue

Hi there,

I just purchased a new Alienware m17x from your website on the 17/01/2013. Just a few days before purchasing the laptop I've seen your offer 'Save £100 (Inc Vat) until 23/1/2013' for the m17x . That's what made me decide that it's the time to buy an Alienware. On the date of purchase, I've noticed that the offer disappeared from the website and with it the m17x with the discount applied disappeared from my saved basket. I decided to buy the laptop at the full price anyway. Now I go to your website and see that the £100 pound discount is available again and I am totally furious and disappointed with my purchase! How can the offer just appear one day, disappear the other and then be available again? I repeat, it says that the offer is valid until the 23/1/2013, but it was not available when I purchased my item, thus making me lose £100. 

My customer number is: