Builders express I want my money back!!!!!!!

I bought a gas cylinder 2 weeks back, they also filled it with cylinder, this was for my gas heater, I got home and it was not working, took it back to builders express sunward and they tested it on the heater on the floor at the store.

Turns out that the fault is my heater and not the gas cylinder, this gas cylinder hasn't been used but they don't want to refund me.

What must I do with a gas cylinder without a heater

I paid over R1000 Surely as a customer something should be done