Hi, My Name is Tas I will be happy to assist you with your question today.
how old is this phone approximately ?
ive had it since the middle of this year.
Ok well that solves the one problem that your phone is still under warranty. Because it is pretty much standard that all the manufactures have atleast one year warranty which starts from the day of purchase.
Second question or problem is how you claim the warranty, if you lost the receipt .. normally manufacturer don't keep the track when the warranty starts because they can't tell for how long the phone was sitting on the shelf.. in that even't it is either your proof of purchase which is the receipt or the store where you purchased normally they have some kind of data base that can help pull the information and send it for warranty repair/claim. I am surprised they don't have this which is very odd. Try to talk to the manager and see if they can hep otherwise the next step is to contact Nokia directly for some help or see if you can track the transaction on your credit card or bank statement that will also be proof and date of your purchase ..
Here is also a website link from Nokia that helps you track if your phone is under warranty
problem is its not that if your phone is under warranty its issue of how you can claim that ..
If you can show them atleast the proof of purchase you should be able to send it for warranty repair through your store. I hope this information is helpful. Please ask for more clarifications if required. if you are satisfied with the help don't forget to hit ACCEPT on this answer. Thanks