It's normal for Zen to take payment at the time if order for the first month's charges for new services together with any one off charges. Like most providers, you pay standing charges in advance and additional usage charges in arrears.
I'm not sure how Zen set the billing cycle for new customers as I've been with them for many years. That said, I would expect a bill after a week or so with a month and few dsys of regular charges plus any call charges from the first few days. Your prepaid sum should be credited to this bill, though I've had prepaid amounts leading to a credit note rather than automatically being applied (if this happens, phone Zen's accounts department, who will sort it out).
Once you get your first bill, that should establish a monthly pattern of billing dates and Direct Debit dates.