Growing pains: How do I start accepting larger payments?

Hi. I have been running a small decorating company for a long while and usually generate revenue in the £10,000s each month. I've just been approached by a client and they want to offer a large project, more than all the work I get from all my clients at the moment but they only want to put a deposit to start and pay the rest on completion. As I usually deal with smaller clients I usually receive all or the majority of the payment upfront.