Most suppliers would not reduce prices at the customer's request without starting a new contract (and Minimum Term). After all, it's unreasonable to expect something for nothing from a commercial firm.
Without any record, either way, of what was was said in the call, it would be hard to prove otherwise, as you have since reaped the benefits.
Have a look at your AOL account here: http://help.aol.co.uk/help_uk/microsites/manageAccou...
and see if it contains anything about your contract dates.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 16 Meg LLU BB