I'd be curious as to why they are approaching you if they already have a supplier. If their current supplier is letting them down all the time or is considerably more expensive then yourselves then that may explain it, but, if they haven't paid their current supplier that would also explain it.
Most of the companies around here have monthly accounts with a 10% admin charge on top of the fare. Also you may wish to apply a set monthly limit until you are happy that the customer will pay you on time and that your cashflow isn't going to suffer whilst you're awaiting payment. Having an account with a customer that promises hundreds of pounds of work each month isn't worth jot if you don't get paid and unless you can afford to credit check these people you're going out on a limb.
Do you have a Business Link or similar in your area? If you do go and see them they will help you. It may also be an advantage for you to attend a small booking keeping course so you know what you are doing in the future. There are lots things out there to help up and coming businesses you just need to look for them and take advantage of what's on offer.
Good luck
