Their website seems to focus on "goody-bags" and "top-ups", but I assume you can create the equivalent of a pay-monthly contract with them? (Through their auto-top up?)
I'm very tempted by their "£10 for a month" goody-bag, but I don't want to be fiddling around with top-ups every month.
You can "pre-load" another £10 bag at any time during the month and it will kick in as soon as the current one runs out.