FootLocker Complaint

I purchased a pair of Nike trainers in my local Footlocker on Sunday, 22 July.  Upon trying them on a few times at home, I realized they were a bit too tight, so at the first opportunity, i.e. five days later, I returned them, requesting a bigger size.  I was flabbergasted when the assistant told me (and this beggars belief) that, because I hadn't kept the box, they couldn't do the exchange.  I tried to reason with said shop assistant, but was getting nowhere, so I asked to speak to someone in authority, i.e. a manager.  

He basically gave me the same mantra, which I obviously wasn't going to accept.  I tried to explain in words of one syllable that nowhere on the receipt did it say "no exchange without original box", and for a shoe shop to claim they couldn't re-sell them because they didn't have a box to put the m in was totally ludicrous.  

Eventually, after much to-ing and fro-ing, the manager told me to come back on Saturday, as their till wasn't working (???) and he couldn't do the exchange that day.  I am venturing back in there on Saturday, but  if I don't have a satisfactory outcome, I will be bringing this to the attention of Trading Standards and, indeed, Which Magazine, as I feel  the public have a right to know about the dreadful customer service on  offer at FootLocker.

I can safely say I will never make another purchase at this store, and in future will buy my sports wear from a reliable store.