My camera is an Olympus. The only function which does not work with mine using a non-Olympus card is the video function. I use Sandisk and Fuji cards with no problem. I get mine from
www.7dayshop.com as I find price and service good.
If you still have your receipt the shop is legally obliged to honour the 12 month guarantee by law.
www.dti.gov.uk/ccp/topics1/fracts/salegoodsact.htm
Take a print out of sale of goods act with you. Normally makes 'em straighten their story quickly.
Unless they can prove you did something silly and that would likely be very difficult for them.