In reply to Simon_87: these kind of work issues can be an absolute pain in the arse, unfortunately many large companies beleive this is an accepteble way in which to behave. If i were you i'd join a union that represents retail shop workers and ask their advice. It will cost you to join a union but, i have found that they can work wonders when negotiating with employers. If you have the confidence to speak to your boss about this issue, give it a go. If you appear to be calm, reasonable, understanding, but firm you might be able to find somekind of compromise.
Good luck, i hope you can work something out.
Zoro.