As you've said, use a credit card. That way you have the protection of the Consumer Credit Act, which entitles you to reimbursement from your credit card company if you do not receive the goods/services you paid for. Some people don't understand these rights, and simply say NO NO NO. That's a rather uneducated perspective.
Cash, No. Cheque No, Debit card, No. I'll agree that much as it gives you absolutely no legal comeback, but a credit card does give you a good level of protection in law. Pay as much as you need to in order to obtain a decent discount, but as mentioned above, question why they would want more than 10hrs paid up front.