What spider has a silver back and an orange crescent on the underneath of body?

It sounds like it could be a Sac Spider or it could be a Dysdera Crotata (usually found under bits of wood. It typically weaves a flap of silk to hide under while waiting to ambush prey such as woodlice. If you google dysdera and look at images and it is the spider you have seen then you should be warned, this spider is particularly aggresive if handled as Bill Oddie famously found out when filming one if his wildlife shows (brave fella that).