expensive LPG gas

crown green bowler

I think the last time I changed our 6 kg propane bottle on a C.C site I think we paid over 21 pounds for the calor lightweight. I have just changed our 11 kg propane bottle here in spain and paid 16.30 euro on todays exchange rate that is 13 pounds 68pence, 
the same gas, so why so much difference in the price.