Contract Change - illegal?

I'll make this as quick as possible.

My contract is due for renewal. I called up wanting to downgrade as I've been paying £60 a month for 1gig and not it's not needed. The lady said she could do me M250 for £34 a month. I said I'd call back on the 13th (today) 30 days before my contract ends with a decision, all ok agreed that was fine and notes on my file.

I called today and Samantha from Teeside has now said I can't have that and it's £41. How can they agree for me to call on the 13th with a decision for £34 or cancel and now change their mind?

Elaine Poole is now calling me in 48 hrs to discuss. 

Any help would be great.