I've also been charged this pro rata charge. I was on 18quid brilliant broadband, and they wanted double but I couldnt pay it so they said dw it will mean you have to pay the 25from now on, so I was ok with that. But this month they carried over the 36quid, had to pay 61 this month..now they want 50 next month! Double due to "pro rata charges" I'm not happy, I will be going elsewhere if they expect 50quid a month for standard wifi!
