Just want to pick some brains...

I have an ISP account with Mweb and I want to cancel it but they insist on a calender month (30 days) notice. My latest invoice is for 01 to 30 Sept. 2013, so I do not see why the cancellation cannot be done as for 30 September 2013.

It's a bit odd that it didn't take me a calender month to sign up for the service and I'm well within a billing period and paid-up till end September, where do I stand legally, should I cancel the debit order at end September?