Bought Parallels 10, now can't use installation key on 11

I bought Parallels Desktop 10 for Mac from www.amazon.co.uk here - http://www.amazon.co.uk/gp/product/B00MGJJXY2?keywords=parallels&qid=1441120766&ref_=sr_1_1&sr=8-1 on 17th August. Unfortunately I did not know that Parallels 11 was out and when I tried to install 10 today 11 refused to accept my key, saying it was out of date.

Would anyone be able to assist, please as I can't afford to buy another copy so soon.

 

I really would appreciate some help, after all I am a new customer!