How much should i have been refunded?

I brought a �50 corset from Ann Summers, on the 4th of June, and since then it went down to �30 in the sale. I returned it today, with original receipt and i was only given back the sale price of �30. Since iwork in retail i'm sure i should have got the full price i paid back as i had proof of purchase? please help as i need to know if i'm right before i take this further thanks in advance x