Hmm... Not sure they would change the tariff without incurring some charges. The best thing would be just call and ask - but call a couple of times... unfortunately, people on the other end of the phone can and will give you different stories!
How far into the contract are you? If you're in the last month or two, you might be able to say that another company has offered you a deal (make it realistic!) and you're thinking of going with them when you renew and they should be able to offer you something.