Ham, you are still on their premises. They have just one hour to detain you while the police get there.
No, They can only detain you if they suspect you of shoplifting, and the act of refusing to show them your receipt in and of itself is NOT reasonable cause for them to think you have stolen the item.
They are setting themselves up for a loss in court and being found guilty of false imprisonment and paying you money if they detain you for the simple act of not showing the checker your receipt.
The shopkeeper exhemptions do not give them cart blanche rights to detain you.. they must have a reasonable belief you have stolen or were trying to steal something. Simply refusing to dig out your receipt for the checker to look at is not in and of itself going to place them into exemptions to false imprisonment, and they will lose a law suit should the person detain decide to sue them.